Chapter 4. GIS and Mapping

Transcription

1 Chapter 4. GIS and Mapping GIS is not a computer system for making maps or graphics, but rather, a set of tools for identifying spatial relationships between data features, or across data themes. Therefore, data structure is one of the first major decisions to be made in the GIS process. Most GISs are either raster-based or vector-based, but many can employ both formats. Raster can be transformed (changed) to vector, and vector to raster. 1. Spatial Data Organization A GIS represents a set of real-world objects in a combination of various themes or layers as map features, also known as digital map or digital cartographic data (Diagram #1). In a single view, many themes can be displayed for the same geographic area (Diagram #2). Diagram #1 Diagram #2 Spatial data are stored along with some of their characteristics, called attribute data, in the form of attribute tables. An attribute can be a nonspatial aspect of spatial data, such as name, classification, or color. Attributes come in many forms, including text descriptions, numbers indicating quantities of some sort, and abbreviations or codes (for a name or classification). (Davis, 1996) Selected layers from a GIS database are combined to create a map, known as a composition. In a composition, the choice of layers or themes depends on the needs of the user and on the existing data in the GIS. Compositions may be viewed and stored in a computer, or printed out as maps. 2. Spatial Data Characteristics GIS spatial data allows you to display a variety of visible spatial characteristics (although sometimes seen with difficulty) on a map. Although the eye can estimate many of these properties, GIS provides highly accurate measures. Area and perimeter are characteristics of features, and are easily calculated in most GISs. The eye quickly appreciates the shape of features, but computers need special programming to make spatial form descriptions. Work Site Alliance Community Based GIS 1

2 Spatial relationships between and among features can be important GIS project considerations and are not always apparent to the eye. Distance from one feature to another is available through simple measurement. Also, proximity of features, such as connections or nearness, may be determined through various measures to determine neighborhood characteristics. Time can be an important part of GIS data. The date of the data is meaningful when determining trends or change. A given project may need periodical data updating, and obsolete data may cause confusion. In this case, it is important to specify the studied period. 3. How ArcView Handles Themes Geographic databases are the combinations of the themes for a given area. The themes in a geographic database can be used in concert to solve multiple problems through multi-level analyses (ESRI, 1997). The design of a GIS database has to be flexible. For example, you want to be able to add new themes, and delete old ones. In addition, you want to be able to separate themes to create new ones, and combine themes if they have common characteristics (Ibid). Another important function of ArcView is that the link between raster or vector features in maps is dynamic. That is, there is a two-way relationship between the maps and the attribute tables that hold the information about them. By changing information in a table, the respective mapped feature is automatically changed as well. The true power of GIS is its ability to not only display, but to perform analyses on multiple themes at once. 4. Map Displays Figure 4.1 contains vector data displaying three separate layers of metropolitan Detroit. The bottom layer is the boundary census tract file for the City of Detroit. The second layer is the highway network, and the third layer is the Detroit Public High School System. Each of these layers can be edited and manipulated, which will be discussed in more detail in the exercise. Work Site Alliance Community Based GIS 2

3 Figure 4.1. Detroit Public High Schools (Source: Michigan Metropolitan Information Center (MIMIC) Author: Michael Hart) Figure 4.2 also uses vector data in a thematic map showing the percentage of Detroit residents who voted yes for casino gambling. The geographic boundary for the population is from census tracts. Each of the census tracts has been computed by joining the population table and that of the polling results. The results have been color-coded in shades of green, as seen in the legend. The darkest green depicts areas where the polling results have the highest percentages of yes votes while the lightest green depicts areas with the lowest percentage of yes votes. This is one of the powerful spatial displays of a GIS, allowing an analyst to provide information to the concerned parties in a simple visual. Work Site Alliance Community Based GIS 3

4 Figure 4.2. Casino Gambling Voting Map (Source: Center for Urban Studies, Wayne State Univerity) 5. From Hard Copy To Digital Map We may wish to store, manage, and process in digital format some information of interest that we find reported on a regular map (i.e., drawn on paper). We then have to build a so-called GIS application. The paper map has to go through a conversion, from paper or analog form to number or digital form, which is the computer-readable format. This transfers the map data into the computer. This file in turn must be converted to fit into either a raster or vector GIS. This conversion process is usually called geocoding. There are two major methods to convert hard copy maps into digital format. One is called digitizing, and another is called scanning. 5.1 Geocoding Methods Geocoding by tracing over a map with a cursor is called digitizing. The devices used for digitizing are a digitizing board or a tablet and a cursor. The digitizing board is comprised of tiny wires that run horizontally and vertically inside, forming a grid. The digitizing cursor has an optical viewer with crosshairs that allows the Work Site Alliance Community Based GIS 4

5 user to visually locate a point on the map. The most common cursor has at least 16 buttons and contains a keyboard that can be adjusted to the user s preference. Digitizing converts spatial features on a map into a digital format. To digitize, a map is mounted on a digitizing board and a digitizer cursor is used to trace each map feature. When a point is identified on the map, the user presses a button on the cursor and the tiny wires receive an electrical impulse. Then the computer records the current X, Y coordinate location of that position in digitizer units (usually centimeters or inches). This becomes the X, Y coordinate of the point feature, or one of the points comprising a line or polygon. The second technology used to perform geocoding is called scanning. The drum scanner is most commonly used for map entry. This type of scanner receives an entire sheet map, usually clamped to a rotating drum, and scans the map. Now, however, flatbed scanners are becoming more widely used in this geocoding method. The output of map scanning is an image data file, also called a raster data file. In most cases, these raster data should be converted into vector data formation. This process is called raster-to-vector transformation. 5.2 Digitizing Procedures To convert hard copy maps into digital form by the digitizing method, the operation procedure is software dependent. Regardless of which software you use, however, there are three steps you should follow. Step 1: Use a good base map Digitizing a poor quality map directly affects the accuracy of your digital data, resulting in a misrepresentation of the geographical data. You should use paper documents in good condition, clean, easy to read, and not torn or folded. It is then recommended to copy the paper map onto a stable material if necessary. Step 2: Prepare base map Any features that are selected for digitizing should be marked in advance. The base map is prepared by choosing the control points (usually at least 4 control points for each map) at known locations on the hard copy, such that their coordinate pairs into a real world system, like UTM or State Plane Coordinate System, can be documented from other sources. These control points will then be used to transform the entire set of coordinate pairs from the digitizer units into the real-world coordinate system. Step 3: Digitize your map Before digitizing, the map must be taped onto the digitizing table. First, the control points are digitized and then map features are traced out using the cursor. You should be careful to capture features accurately with a suitable level of detail. Usually points are entered one at a time. Lines are entered as strings of points, and must be terminated with an end-of-chain signal to determine which point forms the node at the end of the line. Areas are digitized as groups of lines that close to form a polygon, with a point (called a label point) within the polygon to identify it. An important note; for the line feature, start/end points, which are Work Site Alliance Community Based GIS 5

6 called nodes, are different than the points between nodes, which usually are called vertex points. 5.3 Data editing In most cases, there are always errors in the output data set that was created by digitizing. The process of correcting the digitizing errors is called data editing. There are two kinds of digitizing errors: node errors and label errors. Node errors include two types of potential errors: pseudo node and dangling node. Pseudo nodes occur where a single line connects itself (an island) or where only two arcs intersect. Figure 4.3. Examples of pseudo node errors. (Source: Arc Online Help) Dangling nodes connect to only one arc, or are the unconnected nodes of dangling arcs. Dangling arcs and nodes are created when digitized arcs stop short of, or extend past, an intended intersection point. Figure 4.4 Examples of dangling node errors. (Source: Arc Online Help) Pseudo nodes do not necessarily indicate an error or a problem. Acceptable pseudo nodes may represent an island or the point where attribute values change along a linear feature (e.g. road type changes from pavement to gravel). In some cases, dangling node may also be acceptable; (e.g., in a street centerline map, dead-end roads are represented by dangling nodes). Label errors occur either when a label is missing or when more than one label point is within the area. There should be a unique label point within each area (polygon) to identify it. The methods of data editing depend on the software you use. 5.4 Data transforming The features from the digitized map are not often in the same coordinate system as the data in other themes that we want to display on the same View document. This is corrected by first setting the control points into some real-world coordinate system, such as UTM or State Plane. Then the entire digitized data set will be automatically referenced in that same coordinate system. Otherwise you must transform the digitizer units, which usually is centimeters or inches, to a realworld coordinate system. This process is called data transforming. Work Site Alliance Community Based GIS 6

7 6. Using Tabular Data In ArcView, any tabular data containing XY coordinates can be displayed on a view as point features. In addition, tables containing references to geographic features such as street addresses can be given XY coordinates in ArcView by geocoding. This means that in most cases you don't need to manually digitize geographic data that is tied to point locations. Instead, you can display your existing tabular files directly on a view. Spatial data sources such as ARC/INFO coverages or ArcView shapefiles have attribute tables containing descriptive information about their features. You can work with these tables in various ways. For example, existing tabular data can be joined to the attribute tables of spatial data sources. This lets you represent the spatial features according to the values they have in the fields of your own tables. It also allows you to select features according to these attributes. NOTE: Spatial data structures are relevant to this discussion because given attribute information is hooked onto the spatial data according to the data structure. Remember that raster cells have specific size and location, and that vectors have length between nodes, and a direction that gives us the ability to use the right and left side of the line segments. 6.1 Working With Event Tables in ArcView Event tables enable you to map data that contains geographic locations but which are not in a spatial data format. For example, you might have a file of customers and want to make a map showing how they are distributed throughout the region. You may have a table of accident information and want to display the accidents with reference to their milepost locations along a route. Or you might have a table with information about bird sightings, with the latitude-longitude location of each sighting as fields in the table. In ArcView these are all examples of events (ArcView Online help). 6.2 Types of Events That ArcView Supports XY event tables These tables contain the exact position of features as X, Y coordinates, such as the location of fire hydrants in a city or the points where soil samples have been taken. X, Y coordinates can be obtained by reading a map, measuring positions on a view, field surveying, using a GPS (Global Positioning System) receiver, geocoding the table that contains address information, etc. The X, Y coordinates can be in any coordinate system and units, such as latitude and longitude or meters. Point event tables These tables contain the locations of points specified as positions on a line. For example, a point event table might record the location of bus stops along a bus route defined by their distance from the start of the route, rather than their XY coordinate location. Example: "At 12.5 miles along Route 10". Work Site Alliance Community Based GIS 7

8 Linear event tables These tables define events as segments along a longer line. For example, a linear event table might record the pavement quality along different sections of a highway, where the sections are defined by distance along the highway. This is also a useful method for recording continuous data, such as the location of speed zone changes along a highway, with the location defined by distance along the highway. Address event tables Address event tables contain location identifiers (street addresses, or any other): customers, suppliers, competitors, branch offices, facilities, crimes, etc. NOTE: In order to add a point or a line to a view, the view must already contain a theme with the appropriate reference system (such as a street network, for example). ArcView uses this reference theme as the base map on which to map the events. In order to add address events to a view, the view must already contain an appropriate theme on which a new address can find a match. Then, ArcView can use this reference theme as the base map for geocoding the table. 7. Join & Link 7.1 Joining Tables The ability to join tables based on a common item is one of the most important functions in database management. Simply stated it allows for non-redundant data storage and simplifies database management (Hutchinson and Daniel, 1997). For example, a data file that contains the inventory of a school district can include an address for every student in the district. A look-up table can then be prepared associating each student record with that student s current grade level. Editing the much smaller look-up table ensures that the grade level changes will be subsequently applied to each student. The two major benefits that derive from the use of joined tables are (1) when changes occur, only one file has to be updated; and (2) because the link between joined tables is dynamic, subsequent views of the joined table will reflect changes after a file has been changed (Ibid). ArcView s join function is particularly powerful, allowing tables from different sources to be joined, stored in the computer, and visualized onto the screen as a virtual table. The source file that is to be joined to the spatial theme attribute table can be any of the table formats discussed earlier. The mechanics of how to perform a join are straightforward, and are covered in Exercise 4C at the end of this chapter. A related concept, namely the relationship between the source table and the destination table, remains to be discussed. In ArcView, the destination table is the table to which the fields from the source table will be appended (Ibid). Typically the destination table is the attribute table for the spatial theme. The results of a join are accurate only if there is a one-toone correspondence between records being matched from the source to the Work Site Alliance Community Based GIS 8

9 destination table; that is, only one unique record in the source exists for each record in the destination. The relationship may be one-to-one, as in the link between a land parcel theme and the associated parcel data. The relationship may also exist as many-to-one, as in the link between a student theme, and the look-up table explaining information about each student (Ibid). 7.2 Join versus Link Exercises If there are many records in the source table that link to the destination table, only the first record from the source table will be joined to the destination table. In this situation, you should be linking rather than joining the tables (Ibid). The link function is used when a many-to-one relationship exists between a source table and a destination table. Linking the two tables ensures that all records from the source table are associated with, but not actually joined to, the destination table (Ibid). The many source table records will subsequently be available when you query the spatial theme. This will become apparent when you work through Exercise 4C. In that exercise, we see how tables are connected through the link functionality in ArcView, without actually appending the fields of the source table to the destination table. 4A. Editing tables In ArcView, when you edit data in a table (change field values, add or delete records and fields) the source data for the table is modified. This also means that if you add the modified table to a different project, your changes will be reflected. But when changes are made to the appearance of the table (renaming, hiding fields, or changing the fields display width) these changes apply only to the project in which they are made. Step 1: Change table display Now you will make changes to the appearance of the table Children.dbf. Open the project. Your Drive:\chapter 4\ex04a.apr. Make the Children.shp theme active. Open the Attributes of Children table. From the Table menu, choose Properties to open the Table Properties dialog box. The name of the table appears at the top of the dialog box. At the bottom of the dialog box are three columns. The first tells you whether a particular field is displayed or hidden; the second is the field name; and the third lets you rename the field by assigning an alias to it. In the Visible column, click on the check mark by the Av_status, Av_score, and Av_side fields to hide them in the table display. Work Site Alliance Community Based GIS 9

10 Then click in the Alias column and locate the Id field and type User-id as an alias. Click OK to apply your changes. The Id field is now called User-id, and the Av_status, Av_score, and Av_side fields are hidden. Step 2: Edit values and add a record Suppose you find that the reported birthday for the child named Adolph Kevin Anthony is wrong (see record 10, after you sort the Child_n field in ascending order). It should be 02/15/95 instead of 02/14/95. Your are called to correct this error. From the Table menu, select Start Editing. To sort the Child_n field, highlight the field name. Then click on the Sort Ascending button: NOTE: There is also a Sort Descending button as well. Click on the Edit tool. Click in the cell of record no.10 and field Birth, the value is highlighted. Type in the right value: , then press <Enter> key. Now you will add a record to the table. Work Site Alliance Community Based GIS 10

11 From Edit menu, choose Add Record. Scroll to the bottom of the table. A new record is appended to the table. It has zeros in the numeric fields and is blank in the character fields. You can use the same method mentioned above to set values to the new record. Step 3: Add a field and calculate values You may have noticed that the values in the Birth field have an extra digit (9) that is not necessary for our table. Now you will add a new field called Birth_day to the table and set the correct values to it. From the Edit menu, choose Add Field to display the Field Definition dialog box, set the Name as Birth_day, Type as Number, 16 for width, and 0 for Decimal Places. Click OK to add the new field to the table. The new field is active in the table. Click the Field Calculator button. The Field Calculator displays: In the Fields scrolling list, double-click on [Birth] to add it to the text box. In the Requests scrolling list, double-click on - (minus), then type , to make the expression [Birth] Then click OK. Work Site Alliance Community Based GIS 11

12 Now, each cell in the new field contains a date value, which is formatted as yymmdd. From the Table menu, select Stop Editing. Click Yes to save your edits. Step 4: Close and save the Project Make the Project window active. From the File menu, choose Save As to save the project in your personal directory. Then choose Close Project to close the project. Click No to discard changes. 4B. Query tables Step 1: Start ArcView and open a project If necessary, start ArcView. From the File menu, choose Open Project. Move to YourDrive:\chapter 4\ and open ex04b.apr When the project opens you see a view that contains a point theme called Children.shp and a polygon theme Detroit_zip.shp. Each point stands for a child and his/her family house location. Make the Children.shp theme active and open its attribute table. Step 2: Query on a table and display the selected set The first task you should do is to find out the kids who were born in With the table active, click the Query Builder button. Work Site Alliance Community Based GIS 12

13 The Query Builder dialog box displays: Now you will use Query Builder to select records in the theme table and the corresponding features in the view. In the Fields scrolling list, double-click on [Birth] to add it to the query text box. From the logical operators, click the >= button. Then type Finally you should build an expression like as ([Birth_] >= ) and ([Birth_] <= ). Click the New Set to select all children who were born in Make the table active by clicking on its title bar, then click the Promote button to move the selected records to the top of the table. Work Site Alliance Community Based GIS 13

14 On the left side of the tool bar, you see the total number of children satisfying the condition of the search (in this case, 2 records out of 150). Also, the selected records in the attribute table and the corresponding points in the view become highlighted. Step 3: Summarize a table By summarizing a table, you count the occurrences of each unique value in a specified field. Optionally, you can get statistical information on another field in the table. The results of the summary are stored in a new table that ArcView creates. Suppose your are asked to find out how many children are in each zip area. Make the Attribute of Children table active, and click the Select None to clear any records that have been previously selected. In the table, click on the Zip field to make it active. Click the Summarize button box. to open the Summary Table Definition dialog button In the Save As input box, change the pathname to that of your personal directory, such as Your Drive:\yourname\sum1.dbf. Set Field: as Shape, and Summarize by: as Merge. Click OK. Work Site Alliance Community Based GIS 14

15 A new table, called sum1.dbf, displays. It shows you the number (Count) of children that belong to each zip area. In our example, it shows that one record is not assigned a zipcode. If needed, we can identify this record (by sorting the zipcode field in ascending order), and perform the desired editing. Step 4: Save and Close the project Make the Project window active. From the File menu, choose Save As to save the project in your personal directory. Then choose Close Project to close the project. Click No to discard changes. 4C. Join and link tables Step 1: Start ArcView and open a project (same procedures as the corresponding section in Exercise 4b, but open ex04c.apr). Step 2: Add a table to the project From the Project menu, select Add Table. In the Add Table dialog box, move to Your Drive:\chapter 4\data Click on children_plus.dbf to highlight it. If you do not see children_plus.dbf, make sure dbase is selected in List Files of Type. Click OK to add the table to the project. Step 3: Join tables In ArcView, the join tables operation temporarily appends the fields of one table (the source table) to another table (the destination table). To join two tables, the tables must share a field of common information. ArcView uses the values in this field to match the records in the two tables. It is important that the relation of records between the destination and source tables be one-to-one or many-toone. Work Site Alliance Community Based GIS 15

16 Now, you will join children_plus.dbf to the Attributes of children table. Make the children_plus.dbf active, check all its fields and click on the field Child_n to make it active. Table children_plus.dbf is the source table in this exercise. Open the Attributes of children table and make it active. Make the field Child_n active. Click the Join button. Now, do you find any changes of the Attributes of children table? From Table menu, choose Remove All Joins, and notice the changes to the table. Step 4: Link tables When the relation between your destination and source tables is one-to-many or one-to-one, you should link rather than join the tables. Like a join, a link requires that the two tables share a common field. When tables are linked, selecting a record in the destination table selects all records in the source table that have the same value for the common field. Now let s link the Attributes of children table to the children_plus.dbf table. Make the Attributes of children table active and make the field Child_n active. Make the children_plus.dbf active and make the field Child_n active. From Table menu, select Link. (The two tables are now associated, but no fields are appended to the destination table and the source table remains open). Make sure the Select Record tool is available, and click on it. Work Site Alliance Community Based GIS 16

17 Select any record in the children_plus.dbf table. You will find that the corresponding record(s) are selected and highlighted in the Attributes of children table. With the children_plus.dbf table active, from Table menu, select Remove All Links. Using Select Record tool to select any record in the children_plus.dbf table. No record(s) in the Attributes of children will then be selected. Step 5: Save and Close the project Make the Project window active. From the File menu, choose Save As to save the project in your personal directory. Choose Close Project to close the project. Click No to discard changes. 4D. On-screen digitizing In this exercise, you will understand the basic concepts of map digitizing through screen digitizing - a virtual digitizing environment and process. Step 1: Start ArcView and open the exercise project If necessary, start ArcView. From the File menu choose Open Project. Move to the directory Your Drive:\chapter 4\ and double click the project ex04d.apr. When the project opens, you see a view containing the Detroit_zip.shp theme. This time, we will use it as the source data to be digitized into the computer. Step 2: Create a new shapefile as a theme From the View menu, select New Theme to create a new theme. In the New Theme dialog box, select Polygon as feature type, then click OK. Work Site Alliance Community Based GIS 17

18 In the New Theme file browser, move to your personal directory and save the new shape file as Zones.shp The new shapefile theme Zones.shp is added to the table of contents in the View document, and is ready for editing (note the dashed line around its check box). Step 3: Set the snapping environment With the use of the snapping environment in ArcView, you can improve how new features meet and align as you create them. (For more details on the Snap function, please consult the Online Help for ArcView). Click on the Zoom In tool and then drag a box in the area of the view to zoom in a small area (the smaller the area you zoom in, the smaller the snap tolerance can be). Place the cursor in the obtained view and hold down the right mouse button to display the popup menu. Move the mouse down to highlight Enable General Snapping, then release the right mouse button. Click on the General Snap tool Place the cursor in the view. Click and drag out the circle attached to the pointer (as the size of the circle varies, you can read the radius values in the bottom left side of the ArcView screen). You stop dragging the circle once you read the desired radius value (e.g., 0.01 miles) that then becomes the general snap tolerance. Work Site Alliance Community Based GIS 18

19 Step 4: Add features and attributes to the new theme If necessary, use Zoom In tool and Pan tool to zoom the Detroit_zip.shp theme to the desired display scale. You will add several polygons to the Zones.shp theme by subdividing the map area into East, Center, and West. For each polygon you draw, you will add the zoning code to the theme attribute table. Open the Zones.shp theme table. Note that there is no record in the listed fields. You will add a new field called Zone to this table. From the Edit menu, select Add Field to display the Field definition dialog box. Fill it as displayed in the following figure. Then click OK. Make the View window active. From the Draw tool drop-down list, choose the Polygon tool. Now move the cursor (Notice that it changes into cross hairs) into the view. Draw a polygon: each click creates a vertex point, double-click to complete the polygon. Work Site Alliance Community Based GIS 19

20 Make Zones.shp the only active theme. Double-click on it and proceed to the setting as mentioned below. By default, the Zones.shp theme has a solid fill pattern. Polygons you add to this theme cover up the source map (Detroit_zip.shp) beneath these new polygons. You should then change its fill pattern from solid to transparent (first white box in the Fill Palette), and a bold outline (select 2 in the Outline dropdown list, in Fill Palette). Make the theme table active. A record has been added for the polygon you just drew. Click the Edit tool, then click on the record under the Code field and type East as the zone name. Next, you will draw a new polygon adjacent to the previous one (for Center zone). Make the view active. From the Draw tool drop-down list, click on the AutoComplete tool. This tool lets you draw a line to append a new polygon to an existing, adjacent one. Draw a line starting at (or inside) the boundary of the existing polygon (code East). Click to enter each vertex; double-click to end the line at (or inside) the boundary of the existing polygon. NOTE: The two polygons will share a common boundary; you do not need to draw this boundary. Now make the theme table active and enter Center as the second zone name. Repeat the above steps for the last zone that we name West. Step 5: Edit features using Split and Union You will use the Split tool to split the zone coded West into two polygons, then join one of them to the polygon coded Center. Before you edit the polygons, you will specify how ArcView will update their attributes in the theme's editing properties. Work Site Alliance Community Based GIS 20

21 Make the view active, with the Zones.shp theme active. From the Theme menu, select Properties to display the Theme Properties dialog box. Click on the Editing icon. In the Attribute Updating portion of the dialog box, click on the Field drop-down list and choose Zone, then click OK. Note: The Zones.shp theme table should still be open so you can see how attribute values are updated when you perform the split. With the view active, click on the Draw tool drop-down list and select the Polygon Split tool. Click once to start the line. (The line should start and stop outside -overshoot- the boundary of the polygon you want to split.) Double-click to end the line. The polygon is now split into two polygons. Next use the Union Features tool to merge one of these new polygons with the polygon coded Center. Click on the Pointer tool, then click inside the polygon center to select it. Hold down the <Shift> key and click inside the polygon adjacent to polygon Center. Now two polygons are selected. With the view active, click the Edit menu and select Union Features. Work Site Alliance Community Based GIS 21

22 The two polygons are now merged into a single feature with a signal record in the theme table. Step 6: Stop editing and save your edits From the Theme menu, select Stop Editing. When you are prompted to save your edits to Zones.shp, click Yes. Step 7: Save and close the project Activate the Project window and, from the File menu, choose Save Project As. Move to your personal directory and save the project as a new project name. From the File menu, choose Close Project. 4E. Heads-Up Digitizing and Creating Data In this exercise, you will learn how to create your own data by digitizing off a topographic map that has already been brought into ArcView and given a projection and coordinate system. Digitizing, as described earlier in the chapter, is a way to get data from a paper map into the GIS. Sometimes there will not be sufficient data for a project and some will need to be created. You are going to learn how to create your own data using heads-up digitizing. Heads-up digitizing is similar to tracing off a base map. It does not involve using a digitizer tablet or special mouse; just the ArcView program and a base map off of which you will digitize. Work Site Alliance Community Based GIS 22

23 Step 1. Start ArcView and open the exercise. From the File menu choose Open Project. Move to the directory: Your Drive:\Chapter 4\ and double-click the project file ex04e.apr. When the project opens, you see an empty view. Click on the Add Theme button and navigate to Your Drive:\Chapter 4\data Under the Data Source Type select Image Data Source. Highlight Ypsi_east.tif and click OK. The image will be added to the View. Click on the Ypsi_east.tif theme to make it active. Notice the different colors associated with the theme. The major highways and roads in the theme are red, lakes and ponds are blue, urban areas are reddish. Zoom in to the area of the map that is urban. Notice the schools, hospitals, and post office are points on the map as seen below. Work Site Alliance Community Based GIS 23

24 Step 2. Create a New Line Theme To create a new theme, you first must decide whether it is to be a point, line, or polygon feature. Let s digitize the roads first, so that would be a line feature. Under the View Menu, choose New Theme. A box will display asking which type of theme you want. Select Line. Work Site Alliance Community Based GIS 24

25 Give the new theme a name and put it in your personal directory: Your Drive:\yourname\roads.shp. The new roads.shp theme has been added to the view. The box around the checkmark means you re in edit mode and can begin digitizing: Click on the Draw Line tool. The cursor will turn to crosshairs. Begin digitizing along highways and those roads depicted with a heavy red line at one edge of the map by moving the cursor along the feature, clicking periodically. When you are digitizing, it is important to be accurate. Zooming in and zooming out is necessary to see the details as well as the bigger picture. Click one time and then move the mouse and click again. Continue clicking until the line is complete as shown below. Then, double-click to end the line. NOTE: If you zoom in few times it will make the digitizing easier. But this means that actually following the road is hard because it is segmented. When you have begun digitizing, right-click on the mouse to access the following menu: Select Pan, this will move the base map over so you won t need to stop and start as much, giving you a continuous line. Also in this menu is the Delete Last Point Work Site Alliance Community Based GIS 25

26 tool. If you decide that the last point you digitized is not correct, right-click and select Delete Last Point. You can use this feature a number of times in a row to delete more than one point. There are also options for Zoom In and Zoom Out. You can right-click in the middle of digitizing a line to make sure you are in the correct location. By using this right-click menu, the digitizing process is not interrupted, leaving the data fragment free. Continue digitizing more of the highways on the rest of the base map. Periodically, save your edits by going to the Theme menu and selecting Save Edits. If you digitize all of the major highways, the roads.shp theme will look similar to the one below: Under the Theme menu select Stop Editing. Select Yes when asked to save your edits. The roads.shp theme is complete. Now we need to add values to the attribute table associated with roads.shp. Click on the Open Theme Table button. Under the Table menu select Start Editing. This enables us to make changes to the table. Now we need to add a field to identify the different roads that were digitized. Under the Edit menu choose Add Field Make the following changes in the Field Definition Box: Name: Name Type: String Number: 16 (default) Work Site Alliance Community Based GIS 26

27 The new field Name is added to the table. Return to the View and select one of the lines. Return to the Attributes of roads.shp by clicking Open Theme Table. There will be one record highlighted. Click on the Edit tool and enter the name of the road into the Name field. Select another road in the View and add the name to the new field. Continue this process until all records contain names. Other fields can be added to the Attributes of roads.shp table. A field name Type can identify the different types of roads (i.e. interstate, primary, secondary, etc.) Add more fields on your own for practice. Select Stop Editing under the Table menu. Work Site Alliance Community Based GIS 27

28 Step 3. Create a New Point Theme The area south of Ford Lake in Ypsilanti Township is steadily growing. Many new homes are being built in large subdivisions. Let s use these new homes as our point theme. Under the View menu, select New Theme but this time we will choose point for the Feature type, to digitize new buildings. Click OK. Give the new theme a name and put it in your personal directory Your Drive:\yourname\buildings.shp. Double-click on the buildings.shp to open the Legend Editor to change the legend from a dot to a square. Choose a square dot instead of a circle by double clicking on the symbol in the Legend Editor. Choose a different color. Be sure to click Apply before closing the Editor. Find an area south of Ford Lake that has purple points. This indicates new development since the last time the topographic map was updated. Work Site Alliance Community Based GIS 28

29 Zoom in closely to the purple subdivision: Choose the Draw Point tool and begin digitizing the different houses in the new subdivision. Continue to digitize all of the houses, remembering to save edits periodically under the Theme menu. When you are finished digitizing, choose Stop Editing under the Theme menu. The buildings.shp theme is complete. We can add the addresses of the houses we digitized to the Attributes of buildings.shp. Open the table and start editing. Add a new field under the Edit menu. Make the following changes in the Field Definition Box: Name: Street_Address Type: String Number: 25 Work Site Alliance Community Based GIS 29

30 In the View, choose one of the buildings that you digitized. Return to the Attributes of buildings.shp and enter the Street_Address field. For the purposes of this exercise, arbitrary addresses are entered. Continue to select different buildings and enter addresses. Other fields can be added to the Attributes of buildings.shp table. A field name Type can identify the different types of buildings (i.e. commercial, residential, industrial, etc.) Add more fields on your own for practice. Work Site Alliance Community Based GIS 30

31 Select Stop Editing under the Table menu when you have finished adding addresses to the table. Be sure to save your edits. Step 4: Create a New Polygon Theme For this step, we will be digitizing Ford and Belleville Lakes. Under the View menu, select New Theme and choose polygon for the Feature type, to digitize the lakes. Click OK. Give the new theme a name and put it in your personal directory Your Drive:\yourname\lakes.shp. Zoom in to the area surrounding Ford Lake. Select the Draw Polygon tool. Begin at the northern part of the lake where the Huron River enters. Follow the darker blue boundary around the lake until you reach the starting point again. Start a new polygon for Belleville Lake. Once you are done digitizing, save your edits and select Stop Editing under the Theme menu. Open the Attributes of lakes.shp so we can add a new field to name the lakes. Select Start Editing under the Table menu. Add a new field under the Edit menu. Work Site Alliance Community Based GIS 31

32 Make the following changes in the Field Definition Box: Name: Name Type: String Number: 16 (default) We only digitized two lakes in this theme. In the View theme, select one of the polygons. Return the Attributes of lakes.shp and enter the name of the lake in the new field. Repeat this process with the other lake. Other fields can be added to this table, including lake depth, area, perimeter, etc. Select Stop Editing under the Table menu when you have finished adding the names of the lakes to the table. Be sure to save your edits. NOTE: To add more points, lines, or polygons to the themes, select Start Editing from under the Theme menu. Work Site Alliance Community Based GIS 32

33 Now that we have digitized a point theme (buildings.shp), and line theme (roads.shp), and a polygon theme (lakes.shp), take a look at them together: Step 5: Save and close the project Activate the Project window and, from the File menu, choose Save Project As. Move to your personal directory and give the project a new name. From the File menu, choose Close Project. Work Site Alliance Community Based GIS 33

34 Bibliography Davis, David, GIS for Everyone. ESRI Press, 156pp. Environmental Systems Research Institute (ESRI), ArcView Online Help. Environmental Systems Research Institute (ESRI), ARC/INFO Online Help. Michigan Metropolitan Information Center (MIMIC), Michael Hart, author of Detroit Public Schools Map. Hutchinson, Scott and L. Daniel, Inside ArcView GIS. Third Edition. On Word Press, 544pp. Wayne State University Center for Urban Studies, Work Site Alliance Community Based GIS 34