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I have saved Google Maps image (not Google Earth) of area at latlon(17.692437,83.218689) by some method in jpeg format. It is an exact snapshot of the Google Maps. Now I need to geo-reference it to use it as base map for my digitization. What projection or coordinate system shall I use? Some says its WGS-84 Mercator projection. But I see no Mercator projection in GCS list in ArcGIS. Please help! I am confused.

I want to georeference it by assigning exact latlon values to the points as in google maps.

Extend : Now I got another question. Is Google Maps in Projected Coordinate system or Geographic Coordinate system. Though it displays LatLon values, I think it is projected coordinate system. Am I right?

Extend

I set the projection of ArcMap Dataframe to Projected Coordinate System ( Web Mercator) and changed the display values from Meters to Decimal Degrees. Then I added the Google Map with no geo-referenced data to dataframe. Then I took some common control points in my ArcMap image and Google Maps, set the Latitude and Longitude values from Google Maps in ArcMap image points, After assigning latlon values to 4 points, I click on update georeferencing. Then my image is transformed very strange. Is the procedure I did correct?

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Are you sure what you are doing is legal? Google's TOS may prohibit this. See question 8 on this page: google.com/permissions/geoguidelines.html –  blah238 Feb 18 '13 at 17:41
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2 Answers 2

Google uses WGS 84 Web Mercator as its coordinate system. In ESRI it is under geographic coordinate systems.

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But Web Mercator is Projected Coordinate system? –  Jeevan Patnaik Feb 18 '13 at 16:37
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Yes, it is projected, with their own method ;-). They display lat/lon coordinates, but a map in lat/lon would look rather distorted if you come north. If you zoom to world extent, you see that Greenland is much too big. Thats typical Web Mercator. –  Andre Joost Feb 18 '13 at 18:03
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Internally Google Maps is using a projected coordinate system (spherical Mercator), but when you query for or pass in coordinates, the API returns lat/lon. –  mkennedy Feb 19 '13 at 17:09
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The first thing you want to rule out is if the jpegs are spatially referenced or not. To do this:

  1. set your dataframe to the projection/coordinate system you believe the raster is in
  2. add the raster to the map
  3. add data that you know is correctly spatially referenced (e.g. ESRI basemap, or streetmap data...etc)
  4. if the aerial raster aligns with the basemap data then you probably figured out it's spatial reference. You can then use the Define Projection tool to assign spatial reference to the aerial rasters......DONE!

If the raster does not align with anything than you will have to use the georefeencing toolbar to georeference it. At this point the destination projection does not matter as long as you are georeferencing it to a layer that is correctly spatially referenced. After you have georeferenced the aerial you may have to use the Define Projection tool to assign spatial reference to the aerial that the data frame is defined as.

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