Google Earth and Illustrator for Geologic Mapping
If you don't have access to ArcGIS, Google Earth (aerial photos + 3D topography) and a transparent 24k topographic map (standardized base) can provide a very serviceable base for geologic field maps. I use Adobe Illustrator to make my maps, so my linework, symbology, cartographic element, marginalia, etc. is all done there.
There are a few tricks to get consistent results. Returning to the same scale can be a pain. Here's the way I do it.
- Download the 1:24,000 scale USGS topographic map .kmz file from the U.S. Geological Survey's topoView website: https://ngmdb.usgs.gov/topoview/
- Click View and Download Maps Now button.
- Click the red, round "24K" button in the right panel.
- Navigate and zoom to your mapping area. Click to highlight the corresponding map.
- Select the version of the topo map you want in the list at right. I tend to choose maps older than 2010. I prefer hand-drawn contours.
- Download the .kmz file. KMZ and KML files are native formats for Google Earth. For example, I'm downloading "Mt. Harding, MT from 1994" .kmz, which is 3 MB.
- A compressed .zip should appear. In my case its called "MT_MountHarding_265576_1994_24000_geo_kmz".
- On a Mac, double-clidk to unzip it. The .kmz file should appear (MT_Mount Harding_265576_1994_24000.kmz). You can delete the .zip now.
- Open Google Earth.
- Drag and drop the .kmz onto "My Places" in the left menu.
- The map should appear in the correct location in Google Earth.
- There will be 2 subfiles associated with the .kmz. One is just a placemarker, centered in the map outline (turn this off). The other is the actual map (this one is important). In my case, its actually called "Map".
- Right-click the map subfile in the Google Earth menu > Get Info > Description tab > Drag transparency slider to a position about 1/3 position from the left end. I put it right on the line between "Description" and "View". Click OK. See example below.
- Maximize the window. On a Mac, click the green button in upper left corner. This is important.
- Create a placemark near the center of the quadrangle. Its location is not crucial. Before you close the window, click the View tab > Set Range to 1500m. This value sets the eye height, thus the map scale in the view. It needs to be consistent. You'll adjust this value in the next step to better meet your mapping needs. Click OK.
- Right-click placemark > View > Range...Set to a value that makes sense for your mapping needs. Click OK.
- Collapse the Google Earth sidebar.
- Set vertical exaggeration: Google Earth Pro menu > Preferences > Set vertical exaggeration scale to 0.01. Click OK.
- Without changing the zoom scale with mouse, pan methodically across the map area capturing slightly overlapping screenshots. Save all to your desktop or a new folder.
- Open each and crop out border stuff and Google logo. Save each.
- Open Adobe Illustrator.
- Drag the layers into a new AI document. Select all and set opacity to 55%. Arrange all map tiles so they overlap pixel perfectly. Group matched tiles as you go. When done, select all and make opacity 100% (opaque). The image below shows 2 matched tiles on an AI artboard sized to accommodate both.
- Save you new .ai file.
- Lock the image tiles layer. Create a new layer above it for linework. Create another for text. Create another for fills. Arrange layers intuitively. Save your .ai.
- Complete your geologic mapping. Go to the field. Back in the office, use all the 3D functionality of Google Earth to help get linework correct for areas you could not visit in person. Remember to reset the vertical exaggeration to something reasonable.
- In AI, create a polygon of the map border carefully and accurately on its own layer. Trace the border from the new .jpg map image.
- With your mapping completed, return to the topoView website. Download the same quad map, but get the JPEG (or GeoTIFF) this time.
- Bring it into a new AI document. Lock this layer. Save this new .ai. Save your old .ai (your mapping).
- Unlock your mapping layers. Copy and paste them to the new .ai. Don't bring over the tiled images base layer. Scale and rotate your mapping layers to match the JPEG. Grouping and use of shift-scale helps.
- Resize Artboard to closely fit the entire map area.
- File > Export > Export As... .jpg / Use Artboards / RGB color space (flat image file for tracing) or export only the vectors as a .pmg. Make sure to include the map border layer.
- Bring this "unprojected" image (or the arcs layer .pmg) into ArcGIS. The border polygon is your reference for warping the map to match your GIS coordinate system. If you carefully traced the border boundary from the base map .jpg from topView, the coordinate system and projection is already set to whatever that map was made in. Check its metadata in topoView. You should only need to rotate and translate. Worst case, you have to georectify the image using its 4 corners.
The old school alternative method is to use a printed USGS topo and mylar overlay, which is then scanned an brought into GIS. Better to use all the power of Google Earth and