GY461/GEO461 Computer Mapping & GIS Technology Merging of DEM and Geology Raster Images

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1 Introduction In this exercise you will merge the geology color fills created for the quadrangle geologic map, with a hill-shade image produced from a digital elevation model (DEM) of the quadrangle. This merged image displays the relationship between geology and topography. Some quadrangles will display this relationship more strongly than others, nevertheless, there is usually some correlation between the topography and geology. For example, a sandstone composed of predominantly quartz will be a ridge-former simply because it weathers more slowly than surrounding rocks. STEP 1: Creating the DEM Hillshade raster image A raster image is simply a bit map. The DEM data that will be downloaded is simply a matrix of (X,Y,Z) values representing a grid of elevation measurements. This grid can be mathematically shaded as if the sun were shining upon it to produce a hillshade raster image. You will first download the DEM file from the following web site: From this web site follow the links for downloading DEM 1:24,000 data files. You will need to determine which county your quadrangle falls in to find the DEM file. When the file is downloaded create a folder for your quadrangle- in this example I will use the Millerville, Alabama, quadrangle. Copy the DEM compressed ZIP file to this folder: c:\usgs_dem\millerville\ And then extract the files with the WINZIP utility program. You will see a number of DDF files now in the folder. Now you will use the application Surfer to create a hillshade raster image of the DEM. Start Surfer by double-clicking on its icon on the desktop (it should be installed already on your workstation). Select the menu sequence Map > Shaded relief map and indicate the path to the DEM files you just downloaded. Figure 1 displays the file open dialog using Millerville data as an example. The dialog in Figure 2 follows immediately after the file open dialog- simply accept the defaults by clicking the OK button. You should now see displayed a hillshade map of your quadrangle area based on the DEM. Figure 3 displays the hillshade map for the Millerville quadrangle. The next step will be to export the hillshade created in Surfer to a raster TIFF file. Use the menu sequence in Surfer File > Export to bring up the dialog in Figure 4. Note that the image should Page 1 of 17

2 be sent to your AutoCAD Map folder, in this example E:\Acadmapdata\XXX is used. The name of the file should begin with the quadrangle abbreviation, in this example MI_DEM.TIF will be used. The Figure 5 dialog follows the file open dialog. Accept the default values by clicking the OK button. You should now have raster image of the DEM hillshade in TIF file format. STEP 2: Exporting the Geology Color Fills from AutoCAD Map to a Raster Image The next step consists of exporting the geologic color fills to a raster image file (TIF file). At this time load your AutoCAD Map quadrangle geologic map and turn off all layers except for the LithologicPolygons layer. Figure 6 uses the Millerville quadrangle as an example. Select the menu sequence File > Export form the main menu. You should export this file in the raster BMP format to your AutoCAD directory. Figure 7 displays the export file dialog set up appropriately. Note that the BMP raster file type is selected, and that the file path is pointing back to the XXX example AcadmapData directory. At this point AutoCAD will prompt you to select the features to export with a standard selection tool. I recommend selecting all of the color fills with a crossing window (cr). At this point you should now have two raster images in your home AcadmapData folder. One is a hillshade map based on DEM, the other consists of the color geology fills. Both are bitmaps- i.e. raster files. STEP 3: Re-sizing the raster files The Hillshade and Geology raster image files must first be sized so that they dimensionally match each other. This requires the use of a raster image editor application. We will use Paint Shop Pro 7 (PSP), but other applications could also be used. Start PSP from the desktop and then used the menu sequence File > Browse to point to your AutoCAD home folder. After a few moments you should see thumbnail images of the two recently created raster image files. Load each image into PSP by double-clicking on each thumbnail image. After viewing each image you will note that there are margins of unwanted information around each image file. In fact we will need to trim this unwanted information from both files. For both files use the selection rectangle (dashed rectangle tool) to carefully draw a cropping rectangle around one of the images. Note that the image is tilted anticlockwise in this case so it is best to anchor from the NW to SE corner. Use the zoom tool zoom in and precisely record the (x,y) coordinates of the corner points of the image. For example, for the geology color fills the corner coordinates are X Y NW: SW : NE: Page 2 of 17

3 SE: Therefore, the NW corner of the cropping rectangle would be (252, 8), and the SE cropping corner would be (908,788). Use the selection rectangle to set these diagonal corners of a cropping rectangle, and then select Image > Crop to selection to trim the image. Save the image at this time after inspecting it to make sure the cropping operated correctly. Do the same for the hillshade image and also save it at this time. Comparison of the dimensions of the two files in PSP will reveal that the raster files are not equivalent in size. For example, when the geology raster file is loaded into PSP, the application reports the image size as 660x782 (see lower right status line). The hillshade raster is 1509x1793. With the geology raster active, select the Image > resize menu option. Figure 8 displays the corresponding dialog for this example. Note that the maintain aspect ratio is checked (on). Selecting the OK button will proceed to resize the entire image. Note that the y dimension of the geology raster is slightly different than that of the hillshade, but not greatly so. Save the results. Now the two raster files are essentially the same dimensions (or nearly so). STEP 4: Performing the Merge Operation Now the two raster images will be merged by performing bit-wise arithmetic on the two raster files. In this case PSP is used, but other raster editor programs could be used to do the same procedure. Make sure both raster images are loaded, and then select Image > Arithmetic. Figure 9 displays the corresponding dialog. Note that the averaging option is selected. After clicking OK a third image is generated that is the averaged result of the original two. Note the correspondence of geology with ridges. Note how the fault truncates the quartzite ridges. Figure 10 displays the merged results of the Millerville quadrangle. STEP 5: Importing Merged Image back into AutoCAD Map File The newly created merged geology and hillshade image is most useful if it can be imported back into the original AutoCAD Map drawing file. When imported the raster file can then serve as a background raster layer on top of which other line work is plotted. Load your original quadrangle map into autocad Map and create a new layer named DEM_Geology_Composite. Make this the current layer. Make sure that the current units are inches. From the menu select Insert > Raster, which will activate the dialog in Figure 11. Locate the path of the newly created merged raster image using this file open dialog. After selecting the OK button, another dialog is displayed as in Figure 12. Set the options as in this example and then click the OK button. You will next need to set an insertion point with the mouse, at which time the image will appear in the AutoCAD edit window. The location of where you click does not matter because Page 3 of 17

4 the image will need to be moved and scaled regardless. Zoom window around the SW corner of the newly inserted composite image. Draw a point with the point command on top of this corner. Zoom in to make the point as accurate as possible. Do the same fro the NE corner of the inserted image. Now zoom extent so you can see all of the file. Start the move command and select the image and the SW and NE points. When the move command requests a base point, use a node snap to specify the exact center of the SW corner of the image. For the destination use a end snap to specify the exact SW corner of your original AutoCAD quadrangle. You should now see that the SW corner of the image file and the border of the quadrangle coincide. Use the menu sequence Tools > Display Order > Move to back to move the image behind the line work. The final procedure is scaling the image to match the original (correct) dimensions. To do this we will use LISP statements to store measurements and make calculations (although you could do the same thing by hand). The statement: (Setq d1 (getdist)) will store in the variable d1" the distance between the two point indicated after this statement is entered, so enter the above statement and use a node snap mode to select the SW and NE corners of the image. Use a similar statement to set the contents of d2" to the distance between the SW and NE corners of the original map (use end snap). The statement: (Setq sf (/ d2 d1)) calculates the ratio of the diagonal of the original map to that of the image map. This is in fact the required scale factor needed to scale the image to the same size of the original map. Now start the scale command. Select the SW and NE points of the image and the image itself as the objects to scale. For the base point indicate the SW corner of the image file using a node snap mode. When prompted for the scale factor use!sf to recall the calculated scale factor. Zoom to the NE corner of the quadrangle to verify that the scaling worked properly. Remember that you will not have a perfect match, but the image file should agree to within approximately 100 meters. Figure 13 displays the final results. Page 4 of 17

5 Figure 1: File open dialog for loading DEM data into Surfer. Page 5 of 17

6 Figure 2: Dialog for creating a hillshade raster image from DEM data within Surfer. Page 6 of 17

7 Figure 3: Hillshade image of DEM data created within Surfer. Page 7 of 17

8 Figure 4: Exporting hillshade from Surfer to a TIFF raster image file. Page 8 of 17

9 Figure 5: Default values for creating TIFF file of DEM data within Surfer. Page 9 of 17

10 Figure 6: AutoCAD geologic map with only color fills layer turned on. Page 10 of 17

11 Figure 7: File export dialog within AutoCAD Map. Page 11 of 17

12 Figure 8: Resize image dialog within Paint Shop Pro 7. Page 12 of 17

13 Figure 9: Image arithmetic dialog within Paint Shop Pro 7. Page 13 of 17

14 Figure 10: Merged hillshade and geology raster image. Page 14 of 17

15 Figure 11: Dialog activated by the AutoCAD map insert > raster menu sequence. Page 15 of 17

16 Figure 12: Dialog for inserting a raster image in AutoCAD Map. Page 16 of 17

17 Figure 13: Final results of merged image imported back into AutoCAD Map. Page 17 of 17