Lab 6: Transforming Spatial Data

Transcription

1 Lab 6: Transforming Spatial Data Objectives: The primary objectives of this lab are to georeference a raster dataset and to create GIS data by digitizing features from an image as a backdrop. Specifics: There are five parts to the lab. In part 1 you will georeference an aerial image of Washington, DC. In part 2, you will set the coordinate system. In part 3, you will trace the shore line of the Potomac River and the Tidal Basin. In part 4, you will trace the outline of three buildings on The Mall. In part 5, you will add labels and export the map. Part 1. Georeferencing Image Create a lab6 folder in U:\classes\gtech201\labs\. Download and extract your data to the lab6 folder. Navigate to your lab6 folder and click on the file named Lab6.mxd to open it in ArcMap. You will see an aerial image of Washington, DC. At this stage, the image is not georeferenced. The GIS has no idea what part of the world it is looking at. It could be looking at a city in China, Australia, or the middle of the Atlantic Ocean. You will also notice a bunch of yellow lines. These lines correspond to the road network in the DC area. Unlike the image, the GIS does know where the roads are. The trick in georeferencing is to translate an image that is not georeferenced, to match features that are. We will be using one type of translation (rotation, scale change, and transformation) to georeference the image. Take a moment to look at the image in relation to the roads. Can you see road intersections that match with intersections in the image (the bridges might be a good place to start)? To help you see better, you may want to zoom in or out and pan using the buttons on the tools toolbar (fig 1). Required Readings: Open the ArcMap Help and click on the Index tab. Search for the following: 1 Header (.hdr) files > BIL, BIP, and BSQ raster files 2 World files, for raster data 3 Georeferencing, raster data

2 Fig1. Tools toolbar When you see how the roads and the image match up you are ready to georeference the image. To do this right click on any of the toolbars. From the pop up menu that appears select Georeferencing (fig 2). Fig2. Opening Georeferencing Toolbar You will now see the Georeferencing toolbar. On the toolbar there is a drop down list box, that shows the name of the layer you are georeferencing (also known as the target). Next to the target list box can be one of three icons: rotation, or scale change respectively. These refer to transformation, If the georeferencing toolbar is not set to transformation ( ), do so by clicking on the

3 downward arrow next to the icon and selecting the transformation icon (in ArcMap it is called shift ). When you move the mouse over the image it turns into a hand. You can now move the image by clicking on it and dragging it. When the image and the streets are aligned, save your changes by selecting Update Georeferencing from the Georeferencing toolbar (fig 3). Fig3. Update Georeferencing Part 2. Setting the coordinate system Now that you have georeferenced the image, you want to set the coordinate system. To project the image first move your cursor into the center of the screen, right click and then select Data Frame Properties (fig 4). Question 1: What is the appropriate coordinate system to use for the data frame? How did you arrive at that conclusion? In the Data Frame Properties window, click on the Coordinate System tab. Set the coordinate system and click OK to close the window.

4 Question 2: Is the coordinate system defined for all your vector data layers? If not, you will need to define a coordinate system for those data layers by doing the following: Open the ArcToolbox and click on the Index tab. Type "Define Projection". Click on the "Define Projection (management)" tool. In the Define Projection window, select the dataset with an unknown coordinate system as the input dataset. Under coordinate system, click on the button to open its spatial reference properties. Here you can select a coordinate system definition from either an existing definition list or from another dataset or create a new coordinate system definition. Part 3. Tracing the Potomac Now that you have georeferenced the image and set the projection you are ready to add some features. The first step to do this is to open the editor toolbar by selecting Editor Toolbar from the Tools menu (fig 5).

5 Fig5. Opening Editor Toolbar You then want to start an edit session (fig 6) by selecting Start Editing from the editor toolbar If you see the following warning (fig 7),you have not properly defined the coordinate system for your data layers. DO NOT click on start editing. Go back to Step 2, Question 2. Fig7. Warning message The first layer you want to edit is the layer called Potomac. To make sure you are editing the Potomac layer find the drop down list box next to the word Target in the editor toolbar.

6 From that list box select Potomac. Fig8. Setting Target Layer On the Editor toolbar click on the the sketch tool which looks like pencil icon (fig 9). Start drawing your line by single clicking along the eastern edge of the Potomac river near when the I 66 bridge (the northern most bridge over the Potomac river) (fig 10). Fig 10. Location of features in Washington, DC Trace the river's edge by single clicking along the shoreline. You may want to experiment with zooming in and out (fig 1) to find the image resolution that works best to follow the river s

7 edge. After a couple of points, right click anywhere on the screen to see your range of options. Observe the difference between right clicking away from the line you are drawing (your digitized feature) and right clicking on a point you made (a vertex). You can also use the pan tool while you are digitizing by clicking on the pan button (looks like a hand) moving around the image and then continue by clicking the pencil button again and clicking to make more points along the river. Move your way down the river's edge until you get to the Tidal Basin (fig 10). It certainly gets trickier here! Now continue digitizing by tracing the shoreline along the Tidal Basin. Be careful, the Tidal Basin is very curvy, you will probably need to use more points. When you finish tracing the Tidal Basin, you should end back at the Potomac again. Finish your digitizing by tracing the shore of the Potomac until you get to the end of the image. When you are finished right click on the last vertex and click Finish Sketch (fig 11). Save your changes by going to the Editor toolbar and selecting Save Edits (fig 12).

8 Next you will choose a color for the shoreline that will really make it stand out. To do this first, deselect all features by selecting Clear Selected Features from the Selection menu (fig 13). Next, double click on the line symbol for the Potomac layer in the table of contents (fig 14). You should now see the Symbol Selector Window. On that window you can change the line

9 color and width. When you are done press OK (fig 15). Question 3: Which method are you using to input geographic data? (scanning,digitizing,or headsup digitizing). Part 4. Creating a building layer The second part of this task is to construct new features. Fig 10 shows the location of Mall from the aerial image, and fig 16 is a map of some buildings on the Mall provided by the Smithsonian Institute.

10 Fig 16. Map of the mall (source: Smithsonian Institute) Zoom in to the Mall using the zoom in and zoom out tools on the tools toolbar. As you zoom in you ll notice that the image becomes fuzzier. Try and identify five of the buildings from the map above. Although it might be difficult to identify a building s outline, these are common problems analysts face in entering GIS data. Once you have identified three buildings, the next step is to create the building features. Unlike in step 3, in this step you will be adding to the buildings layer. Make sure that the target in the toolbar is set to Buildings (fig 17). Question 4: What type of feature is the Potomac" layer? What type of feature is the building layer? (point, line, or polygon) Zoom into the building you want to capture. As in part 1 you create a feature by clicking on the sketch tool in the editor toolbar. Click on the sketch tool, and trace its outline as best as you can. If at any time you are unsatisfied with your outline you can right click and select Delete

11 Sketch from the pop up menu. When you are satisfied with your tracing, right click on the image and select Finish Sketch. Next you want to set the name of the building you just created. To do this first open the attribute table for the buildings layer by right clicking on the Buildings layer in the table of contents and selecting Open Attribute Table from the pop up menu (fig 18). The last row in the attribute table is the feature you just created. Enter the name of the building you just created in the Name column. When you are done close the attribute table by clicking on the red x in the upper right hand corner (fig 19). Fig 19. Setting the feature name Save your edits by selecting Save Edits from the editor toolbar (fig 12). Repeat the above steps for three different buildings, saving your edits after each step. When you have created the building features, the next step is to select a good color by double

12 clicking on the square underneath the building layer name in the table of contents (fig 20). From the Symbol Selector window you can choose the polygon color, outline color, and outline width. Find a combination you like and select OK. Now that you have finished editing, you can close your edit session by going to the edit toolbar and selecting Stop Editing (fig 21). Part 5. Adding Labels and Exporting Now that you have created your features and georeferenced your image. You are ready to add some labels and export your map. ArcMap provides 2 views. Data view and layout view. The data view is useful when editing features, or doing analyses. The layout view is useful for laying out your map before you export it. Right now you are in data view. To switch to layout view, select Layout View from the View menu (fig 22). In the layout view, you should see your entire map with its title, north arrow, and scale bar.

13 Using the zoom tools in the tools toolbar, zoom into the map so that all of the features you have added are visible (buildings and the Potomac). Next, label the buildings you have added by right clicking on the buildings layer in the table of contents and selecting Label Features (fig 23). Modify the map to make it your own and add in additional map elements. Once you are satisfied with your map, export it to as a "PDF". Deliverables: PDF map of georeferenced aerial photo and digitized river and buildings, and Word document.