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Shapefiles do not overlay exactly on ArcGIS World Physical Map / Google Earth / World Wind

March 06 2009 | 0 comments
Categories: Map Data

How can I overlay a shapefile onto the ArcGIS world physical map more accurately? I have a shapefile containing the countries of the world. When I load this shapefile into ArcMap with a World Physical Map layer, there is about a 15-20 km easterly offset from the Physical Map. I see this same offset when I convert the shapefile to KML and load it into Google Earth, and when I load it into World Wind. I have played with converting datums but using MapWindow but I have not been able to correct for the offset. The offset seems to be too large for an NAD27 to WGS84 issue, and my shapefile covers the entire world. Stumped in Colorado... Attached is a screen shot from ArcMap.

Mapping Center Answer:

The coordinate system for your shapefile must match the coordinate system of the Physical Map, which if it is our ArcGIS Online, then it is publishing using the WGS_1984 geographic coordinate system.  The problem may be that your shapefile, which may be defined as WGS_1984, really may not be properly so, if it was improperly projected/transformed from another coordinate system.  You'll need to find the history of that dataset, possibly projecting it back to something else (without transforming it) so you can properly project and transform it.

If your shapefile is using a NAD_1927 based coordinate system, then know that you must project it twice to get it to align as closely as it can--The first projection is to NAD_1983, using the NADCON transformation.  The second is from NAD_1983 to WGS_1984, where you'll need to pick the correct transformation:

  • NAD_1983_To_WGS_1984_1 = North America
  • NAD_1983_To_WGS_1984_2 = Alaska - Aleutians
  • NAD_1983_To_WGS_1984_3 = Hawaii
  • NAD_1983_To_WGS_1984_4 = Continental U.S. ITRF94
  • NAD_1983_To_WGS_1984_5 = Continental U.S. ITRF96
  • NAD_1983_To_WGS_1984_6 = Canada - Quebec
  • NAD_1983_To_WGS_1984_7 = Canada - Saskatchewan
  • NAD_1983_To_WGS_1984_8 = Canada - Alberta

This from our Geographic (datum) Transformations--Area of Use document.

Note, that if this second transformation is the only transformation your dataframe needs to use, then you could use on the fly projection, while specifying the appropriate transformation in the data frame properties (coordinate system tab, then click the Transformations button).

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