Lab 3: Digitizing in ArcMap

Transcription

1 Lab 3: Digitizing in ArcMap What You ll Learn: In this Lab you ll be introduced to basic digitizing techniques using ArcMap. You should read Chapter 4 in the GIS Fundamentals textbook before starting this lab, as the chapter covers the basics of data entry and digitizing mechanics. Data are located in the Lab 3 subdirectory, including Boat_Docks.shp, Houses.shp, and Islands_in_River.shp, files with errors that you ll fix, and Lab3_image.img, an image file that you ll use as a data source. All data are in UTM Zone 15, NAD83 coordinates. Manual Digitizing Digitizing is the process of interpreting and converting paper map or image data to vector digital data. In manual digitizing you trace the lines or points from the source media. You control a cursor, usually with a mouse or digitizing puck, and sample vertices to define the point, line, or polygonal features you wish to capture. The source media may be hardcopy, e.g., maps taped to a digitizing table, or softcopy, e.g., a digital image or scanned map. ESRI software allows us to digitize using either hardcopy or softcopy sources. On Screen Digitizing in ArcMap Our practice exercise will involve digitizing a set of features from a scanned photo. Digitizing directly on screen is sometimes called heads-up or softcopy digitizing. Flatbed scanners and digital cameras are common, so softcopy digitizing is a standard procedure. Scanned photos have some geometric distortion that depends primarily on how the photo was taken (flying height, terrain, camera tilt, and other factors). GIS data are commonly entered from scanned photos because we can easily adjust the display scale, zooming in or out as needed. This often reduces both interpretation and digitizing errors. Setting File Properties Digitizing changes the contents of files, so you must have permissions to modify old files, or to create new files in a workspace. Data on a hard drive may be write protected, so that they may not be edited. The computer administrator may have denied access to certain data, or only allow read access, to protect these data from inadvertent modification. This protection sometimes becomes a problem if you wish to edit data, because you need to write data when editing. You might think you can avoid this problem by copying the data to a portable drive, but often the protection properties are transferred with the data. Data on a USB drive typically are not write protected, and if they are, by default you have permissions to change this. To see the permissions, open Windows Explorer and navigate to the hard disk where the data 1

2 are stored (specified by a drive letter). Notice there is a number of files listed. With the MS Windows operating system, you may observe the properties by: Single left-clicking a file Then, right click over the selected file View the popup window, and select Properties, near the bottom This will display various properties for the file (see at right). Note that near the bottom-left of the menu there is a check box under read only. You should make sure (by left clicking in the box) that this checkbox is blank. With the checkbox blank, left click on the Apply button to the lower left of the window. The file will now be write-enabled You may also adjust folder properties, and apply these properties to all files in the folder. To select a folder single left clicking on the folder. Then hold the cursor over the folder, right click, and proceed as above. Remember to set the path names as relative. If you are going to be working across multiple computers, for example, doing some work at the office and other work at home, then you should also be careful to have the data and project saved in or near the same 2

3 GIS Fundamentals: Introduction to GIS location, and to set the project to have relative paths. See Lab 1 for instructions on setting paths for a project, for details on how to set a relative path. Also remember that you should set your default home workspace/folder, and create a default Geodatabase within that home. In summary, before you get started you should: Use Windows Explorer to create a directory (e.g., \Lab3), preferably on your portable USB drive. As described in Lab exercise 1, do the following: Start ArcGIS and create an empty/new project (remember from Lab 1, if necessary select a new, blank map from My Templates ) Save the project to the just created directory (left click on File in the upper left of the main ArcGIS screen, then left click on Save, and navigate to the directory you created above, e.g., \Lab3. This makes it the home directory for this project) Set your path names relative to that directory (remember, File, then Map Document Properties, then check the relative path box at the bottom) Create a new Geodatabase in the home directory (ArcCatalog near the upper left of the main ArcGIS window, then left click on the Home - folder, then New, then File or Personal Geodatabase, then type in a name when prompted) Left click on this new Geodatabase and make it the Default Geodatabase Digitizing Start ArcMap, and create a new, blank document. Select the add Add Data button; navigate to the \L3 folder, and add the image named lab3_image.img to display something similar to the figure right. The image is a scanned True Color photograph of the type often used for land cover and vegetation mapping. Notice how the coordinates change in the lower right corner of the ArcGIS 3

4 window whenever you move the mouse. These are the coordinates for the view, and they have been established from the image data. Also note and practice the zoom (magnifying glass) and pan utilities (hand), activated by the icons on the toolbar, shown with a vertical orientation, at left. Note that the toolbar can also be horizontal, as shown in the figure below. The toolbars can be either vertical, or horizontal, because they are dockable. This means you can grab them with a left click of the mouse, and drag them to another position around the frame. You can also make them free floating, by dragging them out into some part of the window, and then letting go with the left mouse button. At this point your can create the empty data layers within your Geodatabase or as individual shapefile. Both methods are explained below; Shapefile on Page 5 and Geodatabase on Page 7. After creating you empty data layers continue on the bottom of Page 8 at the Digitizing Features section. Create a New Shapefile or GeoDatabase/Feature Dataset/Feature Class Using ArcCatalog Open ArcCatalog by clicking on the icon in the main ArcMap toolbar (see figure at right) This should open a window (shown below). Navigate to the subdirectory containing the data (here L3 or Lab 3) 4

5 Right click on the L3 subdirectory and then in the dropdown window, select New, then Shapefile (see the figure at right) 5

6 This should open a Create New Shapefile window. You have entry boxes for a Name, and a Feature Type (see the figure right). The default feature type will initially come up as point, but you could also select line or polygon as the default types, using the selection triangle to the right of the Feature Type block. Enter something descriptive for a Name, e.g., Ponds. If you leave the default New_Shapefile name you may have difficulty keeping track of your files. Specify a Feature Type of point Left click on Edit to set the coordinate system. The resultant set of windows will be much like the projection windows already covered in Lab 2. Specify Select > Projected Coordinate System > UTM > NAD83> Zone 15N system. You may also specify a coordinate system from an existing layer, via Edit then Import, and navigating to the Lab3_image.img data set, or any existing datasets with the desired coordinate system. See Video Create Shapefile for a detailed demonstration. Create a new shapefile of type polyline, to hold the roads you will digitize, and a new polygon shapefile into which you will digitize fields. Name these appropriately, and assign the UTM NAD83 Zone 15N coordinate system. 6

7 *************** Begin Optional Section *************************************** Instead of using Shapefiles, you may also wish to create a GeoDatabase, the more complex, but powerful data forms that were introduced at the end of Exercise 1. Video: Create GeoDatabase Again, in ArcCatalog, left click on the target directory (Lab3 or L3 here, or what you named your directory on your working drive), but this time select New -> Personal GeoDatabase. This will create a new GeoDatabase in the directory tree of the ArcCatalog window, and the name will be open for editing. Rename it, here I named it ddigitlayers. Note the.mdb extension is automatically appended for you. Right click on the ddigitlayers.mdb in the ArcCatalog window, and then select New, Feature Dataset. This will generate a set of windows, first to specify the Name (use Lab3 Creations or something similar, then right click on Next) The coordinate system (use UTM NAD83, Zone 15 N, then Next) Then the Vertical Coordinate System (choose None, then Next) Then for Tolerances (take the default, then select Finish). If you d like to know what tolerances are, click on the About Setting Tolerances button on final window. this Now we need to create the point, line, and polygon layers we ll digitize into within this Feature Dataset. Left-click on the Lab3Creations feature dataset you just created, displayed in the ArcCatalog window, then select New, and Feature Class. 7

8 This will open a window which allows you to name the new feature class, and define the Type. Here we ll create a point layer named pondsgdb, here named distinctly from the shapefile version of the ponds we ll create, but it could have the same name as a shapefile in the same directory, because this feature class is stored within a feature dataset. Click Next, and it displays the fields and field properties for the data table for the layer. You would type them in a row and select the data type here. For now, accept the defaults, and select Finish. Note you didn t have to specify the coordinate system for the feature class layer. Because it is contained within the feature dataset, it has the coordinate system you specified for that feature dataset. Now create two more feature classes within this Lab3Creations feature dataset, one a line layer named roadsgdb, and a polygon layer named FieldsGDB. When you are done, the feature dataset should look something like the figure on the right in ArcCatalog: Note that the icon shows the type for each feature class, with three points next to pondsgdb, a line icon next to roadsgdb, and polygons next to FieldsGDB. ************ END OPTIONAL SECTION ******************** Digitizing Features Close the ArcCatalog window (click the X in the upper right corner), and add the lab3_image.img to your data frame. Your goal is to digitize points (ponds), lines (major roads), and polygons (selected fields) from this image. See the map at the end of this document labeled Digitized and Corrected features in Stillwater Township, 2006 for the minimum number of roads, ponds and field you need to digitize; you can do more if you wish. You ll need to zoom in and pan around to discover all the features on the 8

9 map. If you color image disappears, select the lab 3_image.img layer, right click and zoom to layer. These steps are demonstrated in the Video: Point Digitizing, and Deleting, Moving Points Add the Ponds, Fields & Roads layers to your ArcMap, either the shapefile, or the GeoDatabase version. Make sure the new, empty point, line, and polygon layers appear in your table of contents. Left click on Editor, Start Editing A window pops up that shows the available layers in a top panel, and the source directories/geodatabases and types in a bottom panel. You select a target layer by left-clicking on it; it should then be hi-lighted in blue. You should click on the About Editing and Workspaces button in the lower left, and read the description. (Note, this Window may not appear if all the layers are in the same directory) Select ponds (shapefile version) or pondsgdb (GeoDatabase version) as your target layer to digitize, and then click O.K. Select Editing Windows and choose Create Features or click on the tool 9

10 A new Create Features window will display (see at right), either free-floating, or docked on the right edge of your workspace. Left-click on the ponds layer as the target for edits, a blue box will appear around the name. Note that the Construction Tools in the bottom panel of the window change as you select different types of features. Make sure you have ponds selected, and it is a point feature, with Construction Tools as shown at right. Find the Editor Toolbar, usually a horizontal series of icons near the upper-left side of the ArcGIS window (see below). The bar may be vertical, it is detachable, and may be set up as free floating, or along the left or right edge. The icons allow you to add or modify features. Note that not all icons work for all features, some are just for points, some just for lines, some only for polygons. Note that you may select a tool by clicking on it in the toolbar, or that some tools are selected by clicking on one of the choices in the Construction Tools menu. Alternate (with successive left clicks) on the Point and Point at end of line icons in the Construction Tool menu, and note how the active tool in the toolbar, denoted by a box around the tool, changes position. Make sure the add point tool is activated. 10

11 Use the pan and zoom tools to zoom to inspect the image. Note that there are several ponds, dark blobs in the fields, often surrounded by a light- and/or brownish-colored ring. Your task is to digitize at least 20 ponds; we ll get to that in a minute, but first a few words about scale. Scale is important when digitizing. Too small a scale magnifies errors, e.g., 1:50,000. Too large a scale is inefficient, e.g., digitizing at 1:100 you will be very accurate but will require frequent panning. The best scale depends in part on the level of accuracy you need, and how the ponds or other objects appear on the images. Right click on the Lab3_image in the TOC and select Zoom to Raster Resolution. Notice you now zoomed to something like 1:1,200. This is probably larger than needed for this Lab. You can directly enter a scale; here use 1:4,000, a good compromise (as shown in a previous Lab, simple type 4000, without the comma or the 1 and colon, in the data view scale box). (Video, Set Scale to Digitize) Pan until there are several ponds visible in the image (remember keep the zoom near 1:4,000. You use the point tool to add points. If it isn t activated, click on the pondsgdb (or whatever you named it) in the create features window, and if need be the Point icon in the Construction Tools window. Note the cursor now changes when you move it over the image, to a hollow arrow with a shadowy point at the end. This indicates your left-clicks will add data. To digitize points place the cursor over a pond, and single left-click. Each click creates a feature. Navigate and digitize at least 20 ponds as new points. Place the point in the middle of the pond by positioning the cursor and left-clicking. If the default point symbols are difficult to see, you can change the symbol color, size, or type by double clicking on the legend entry for the layer in the table of contents, as you would for any theme. Note that when you select other functions during digitizing, e.g., changing the color or size of a symbol, you will often have to re-activate the point data layer (here pointgdb), and click on the kind of feature in the Constructions Tools you d like to add, and then continue digitizing. 11

12 You should left click on Editor > Save Edits every few features, and when you are done digitizing ponds. ArcGIS can crash, and you ll lose any unsaved work. Note the caret tool on the Edit Toolbar. The caret is a selection tool. You use the caret to select and edit existing features or parts of features. When you activate the caret, the cursor appears as a caret, indicating that clicks will select features. You typically select features to either delete, or modify them. Hold the cursor over a feature while the caret is active, and left click on the feature. Note that it changes color, typically cyan, the default selected feature color. A left click and hold allows you to drag a feature, try it with a few points. A left click to select a feature, then a right click over the selected feature will display various options, most important among them an option to Delete the feature. Use the caret to move and delete point features while digitizing. To de-select feature(s), left click on ArcMap main Selection menu and left click on Clear Selected Features. The color should now match the rest of the features. Finish digitizing all your ponds, save your edits, and stop editing (click on the dropdown control in the Editor Toolbar). We ll now digitize line features, into the roads data layer. Digitizing Lines, and Snapping First we must set the snapping environment. As with many actions in ArcGIS, this is a bit complicated. You can use the new snapping tool, or the classic snapping tool. To use the new snapping tool, first: Left click on Customize along the top of the main ArcGIS window, then click on Toolbars. This will display a dropdown list of choices. Find Snapping, and left click on it. This should show a checkmark to the left of the name. 12

13 This display the snapping toolbar, also a floating toolbar, so it may appear anywhere and you may click and drag to reposition it Now click on the snapping dropdown pointer, and then select Options: This will display a window that allows you to set options, most importantly the snapping tolerance. Unfortunately, we can only set it in pixels, the default units it chooses because we have an image loaded. We can calculate the tolerance distance we want, but we have to convert the distance we want (e.g., 4 meters) to the proper number of pixels. If you don t know the units for the image, we can look them up (remember, right click on the image in the TOC, then Properties, then Source). Verify that the image has about a 60 cm resolution, so for 4 meters we d like to specify 7 pixels. Many folks choose Classic Snapping, for among other reasons, because it allows us to specify the units in real-measurements, e.g., meters, and provides a more obvious display of the features participating in the snapping. To activate Classic Snapping, leftclick on Editor, then select Options from the bottom of the dropdown menu, then place a check in the box next to Use classic snapping (see the figure, at right). 13

14 To set the snap tolerance and units, left click on Editor, Snapping, Options (see belwo left) The Snapping Options t allows you to set the snapping tolerance in map units or pixels. To change snapping approaches select Editor, Options and select Classic Snapping. We recommend you use one, or the other, so if you choose to use classic snapping, remember to turn off the Snapping Toolbar (via Customize, Toolbars, and then click again on Snapping to toggle off the check mark). Set snapping for something like 4 to 6 meters. One final step to set up snapping. If you are using Classic Snapping then your need say what layers will be snapped. Left click again on Editor, Snapping, as in the figure at above left, but this time select Snapping Window. This will display the layers, and what features will participate in the Snapping Environment. Left click to check the Vertex, Edge, and End boxes for the roads layer, so that these elements of a feature will snap: 14

15 Now specify the roads layer as our digitizing target, select the roads layer in the Create Features window and then select Line in the Construction Tools window. Video: Digitizing Lines This should activate the line tool in the Editor Toolbar: You ll digitize the set of roads shown in the map at the end of these instructions. Set your scale to something like 1:4000 to 1:8000. Move the cursor to the start of the road you wish to digitize, and left click. Move along the center of the road, left-clicking when you need a vertex. Note that this shows both the vertices (knots on the path) and the thread connecting these knots. This is your digitized line. Double click to end a line. If the entire road segment you wish to digitize is not in the view, navigate around the image using the pan and zoom tool. Note that you first pan or zoom, and then you must click again on the target feature (roads, here) in the Create Features window to return to the digitizing cursor. If you are digitizing and the road continues beyond the edge of the view, the feature is still active, and you can add to it, after you click on the layer again in the Create Features window. You may need to refresh after a pan and zoom, using the refresh button next to the data and layout view buttons: Fixing Mistakes What if you make a mistake while digitizing? Placing the sketch cursor over the last point digitized, and right clicking reveals a dropdown menu, as shown at right. You may then select Delete Vertex, but only to remove the last vertex digitized. If there is a sequence of bad vertices, you can move backward, positioning the cursor and right clicking over each one in turn. If you wish to remove an entire line, you may -terminate the line (double click with the sketch tool to end digitizing), then 15

16 -single left click on the line with the caret tool, then - right click and select Delete from the dropdown menu (not shown). You may select several lines by holding down the shift key, and left clicking on each of them to select, then single right-click and Delete. If you want to reshape a line, either to move or add vertices, you can double left click on the line, which will display a segment editor toolbar: Double left-clicking on a line segment reveals the vertices. If you select the leftmost caret, and hold it over a vertex, you can left-click and drag to move the vertex. The caret-plus and caret-minus icons allow you to add or delete a vertex in a selected segment. Digitize and fix mistakes until all indicated roads are digitized. Remember to set and use your snapping tolerance so you don t have under and overshoots. Save frequently using Editor > Save Edits, and stop editing when you ve digitized the roads shown in the example map below. You can digitize more than the example map roads, but you don t have to. Change your Create Features layer to the empty polygon data set you created, and set the Construction Tool to Polygon. Digitize the fields shown in the example map. Video: Polygon Digitizing Move the cursor to a border of the field you wish to digitize and left-click the mouse. This starts a new polygon. Each time you left-click, you will be placing a vertex for a polygon, until you double click, which closes the polygon. Move the cursor to the next field, and repeat the digitizing process. Continue digitizing features, at least as many as are shown in the sample map at right. Save frequently using the Save Edits option of the Editor Toolbar and Stop 16

17 Edits option when you are done. Editing Shapefiles Add the following shapefiles to your data view: Boat_Docks.shp Houses.shp Islands_in_River.shp Videos: Edit Points, Edit Lines, Edit Polygons Specify a display scale of 1:1500, because we ll need to view these data with a high magnification to edit them. These layers have errors. The houses are not in the correct places, the Islands include water, and the boat docks are unconnected or in the river. You will fix all three, using the tools you learned above. Set the Create Features layer to Houses, and use the caret to reposition the house features to the locations of houses in the images. You may need to add and/or delete house features, using the tools learned above. 17

18 Load and display the Boat_Docks. Set the snapping tolerance, to something like 2 to 3 meters. Start editing, and fix the two errors: Zoom to area of the image which contains the Boat_Docks (Zoom to Layer). Hint: they are on the river at the bottom of the image. - the unconnected docks in the northwest corner (upper left of the boat dock features) - trim the too long dock at the southern end (bottom of boat dock features) Save your edits and stop editing. 18

19 Finally, fix the Islands_in_River shapefile. Load it, perhaps change to a hollow symbology, and set the snapping tolerances. The four errors to fix, starting from the upper left corner are: - delete the spurious island - reshape the island to match the tree/water boundary - remove the overlap, and create one island from the two pieces - split the polygon, removing the water from the middle and creating two distinct polygons. You may create new features or add vertices and reshape features as described above, as well as splitting, reshaping, and other tasks that are accessed through the Editor Toolbar. 19

20 To split a polygon into two parts, first left click on the island with the caret it. to select Activate the polygon split tool Place the cursor to one side of the polygon at the point you want to cut, left click, and add vertices to define a path through the polygon, ending the path on the other side of the polygon, clicking to define the path as you go. Double click to end the cut, splitting the polygon into two polygons, which you may then select, delete or modify. Fix all the errors in the layer, saving and stopping editing when you are done. Result Below is the approximate Key for your digitizing. Digitize the Ponds, Road and Fields and correct the Houses, Docks and Islands, and produce a PDF that looks approximately like the image below. Remember to include a North arrow, legend, title, your name, and scale. 20