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I wanted to tie toegther some data from my database table to an ArcGis map. The table contained an Object_Id, GIS_LATITUDE and GIS_LONGITUDE column. I added the feature layer based on the table (x as GIS_LONGITUDE, y as GIS_LATITUDE) and was surprised when my points, instead of being based in California, ended up just off the African coast.

I looked at the details of each point and found that the decimal fields had been translated to be meters away from 0,0 instead of degrees away. So my point:

-119.789° 36.793°

becomes:

-119.789 36.793 Meters

I'm new to ArcGis for desktop and can't seem to find an option that will change the projection to use degrees. Most frustrating is that both the feature layer and the topographical layer is using the same Mercator projection.

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    If the map was already using a projected coordinate reference system, the Add XY Data tool in ArcMap will default to the data frame's coordinate system. Re-add the data but change that to use a geographic coordinate reference system instead. – mkennedy Nov 14 '14 at 18:06
  • Both are using WGS 1984 Web Mercator (auxiliary sphere). Isn't that a degree based system to start with? – ChargerIIC Nov 14 '14 at 18:24
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    Nope, linear unit-based because it uses a map projection. You'll geographic coordinate systems, North America - NAD 1983 or World - WGS 1984. – mkennedy Nov 14 '14 at 18:26
  • That did it. I was using the wrong projection. – ChargerIIC Nov 14 '14 at 18:30
  • It is very easy to do. We've (Esri) discussed changing the add xy data behavior but haven't changed it yet. I'd rather it default to WGS84 myself. – mkennedy Nov 14 '14 at 18:32
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You are probably looking for the Define Projection tool (under Data Management, Projections and Transformations).

Unlike Project, which will transform from one coordinate system to another (e.g. -119.789° becomes some large number of meters), Define Projection changes the coordinate system for the data itself and is appropriate to use when ArcMap is mistaken about units.

  • No, I think mkennedy has it in her comment at the question. – user23715 Nov 14 '14 at 18:13
  • Define Projection is the approach I use for data that's already been added into Arc but has an incorrect projection assigned. It's also true that the problem can be prevented by assigning the right projection when first importing, but the question asked "an option that will change the projection to use degrees". Best practice is to use a geographic coordinate system in the first place, in future :) – Erica Nov 14 '14 at 18:15
  • But can you Define Projection when the data being displayed are from a table viewed as an XY Events-layer? – user23715 Nov 14 '14 at 18:31
  • I just double-checked it on a CSV file I have and it worked, so I assume it would work on an events layer of any source. – Erica Nov 14 '14 at 18:41
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    @mkennedy should make those comments an actual answer. :) Both answers would be correct, one just fixes the problem during the initial step by doing it over, the other corrects as a second step. Both occur before any analysis. – Chris W Nov 14 '14 at 23:02
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In ArcMap, double click on Layers in the Table of Contents, then within the DataFrame Properties box that opens up, click on Coordinate System and select NAD 1983 (or other suitable). This will fix the coordinate system for all layers, you shouldn't have to worry about reconciling systems across layers ever.

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