Here’s some more Google Mapmaker stuff. I’ve got another one I want to write after this for all you cyclists but no guarantees when I’ll get that done. To my latest problem solving adventure.
When I first started with Mapmaker back in April I mapped most of my college campus using image overlays. Our satellite map is from 2005 and we’ve done a lot of construction since then. So I bought a newer one and used it to trace the building maps. It was not easy getting the imagery aligned. I’ve learned a lot since then.
I’ve been wanting to map some of the local parks as boundaries on the map but it’s really hard to determine the boundaries of a park when that park is just surrounded by more trees that look pretty much the same as the trees in the park.
After days of hunting for local tax maps I finally found the maps for my local area. Of course, they’re all just pdfs, no GIS data and I suddenly had this idea about how to make the image overlay process easier. Because it’s such a hassle to adjust image overlays in mapmaker, I’d just do it by overlaying it on a screenshot of mapmaker in photoshop. Then I’d have reference points that I knew would absolutely match up with google mapmaker.
Step 1: Get a screen shot of the tax map with surrounding reference points, like intersecting roads, and paste it as a layer in photoshop*.
Step 2: In mapmaker go to map view get your POI marker displaying on the map and select the zoom you want to map at. You need to keep some reference points, like intersecting roads, in the view. Get a screen shot of this and paste it on another layer in Photoshop. Remember your zoom level or just don’t monkey with the map until you have your image overlay ready.
Step 3: (optional) Switch to satellite view with the POI marker still displaying, other than that, no panning or zooming. Get a screen shot of this and paste it into photoshop.
So now, you should have 3 layers in photoshop. The tax map or whatever imagery you’re using for the overlay, the screen shot of mapmaker in maps view with your POI, another screenshot of mapmaker in satellite view. The satellite view isn’t necessary but it gives you a little extra information in case you need it.
Step 4: Select the color white with the magic wand on the tax map, make sure contiguous is unchecked so you get ALL the white no matter where it is. Delete the white so you have a tranpsarent line drawing of the tax map on the top layer.
Step 5: Reduce the opacity of your satellite view layer so that it’s just barely visible. Align it with the Maps view layer using the POI for reference.
Step 6: Use scale, and other transform tools to make the Tax map reference points, like roads and railroad tracks match what is on the Maps and Satellite layers. DON’T resize or transform the Mapmaker layers.
Step 7: Save in some web friendly format and upload somewhere.
Step 8: Add it as an image overlay. (You may have to turn on overlays in google labs if you haven’t already. I can’t quite remember anymore. )
Step 9: drag all the reference point on your image overlay to match the mapmaker maps view.
Step 10: Draw the polygon for your boundary.
*It doesn’t have to be photoshop, just an image editor that can handle layers and transparency. Gimp would be great for this too.