A Difference of Projections

If you divide your time between ArcGIS and QGIS, you may have noticed that they don’t use the exact same projections.  For Northern Colorado, for example, ArcGIS offers “NAD 1983 HARN StatePlane Colorado North FIPS 0501″, while the best QGIS can do is “NAD83(HARN) / Colorado North”.  When projection information doesn’t agree exactly, I get very nervous.

Fortunately, we can ask our friends at SpatialReference.org for further information.  By browsing through the list of projections for Colorado, we find that the ESRI projection above is called “ESRI:102253″, and the QGIS equivalent is called “EPSG:2772″:

http://spatialreference.org/ref/?search=colorado

EPSG stands for European Petroleum Survey Group, an international organization which collects and catalogs spatial data parameters.  Their projection definitions are the world standard for everyone except ESRI, who prefers to keep their projections proprietary.  So the question is, what’s the difference?  Which one should I use for my project?  Which one is “better”?

To find the answer, click through to a projection from the above list, and then click on the Proj4 definition, which shows you its parameters quite clearly.  This is the Proj4 statement for EPSG:2772:

+proj=lcc +lat_1=40.78333333333333 +lat_2=39.71666666666667
+lat_0=39.33333333333334 +lon_0=-105.5 +x_0=914401.8289 +y_0=304800.6096
+ellps=GRS80 +units=m +no_defs

And this is the Proj4 statment for ESRI:102253:

+proj=lcc +lat_1=39.71666666666667 +lat_2=40.78333333333333
+lat_0=39.33333333333334 +lon_0=-105.5 +x_0=914401.8289 +y_0=304800.6096
+ellps=GRS80 +units=m +no_defs

As you can plainly see, the definitions are largely identical.  The only difference is that the 2 standard parallels that define the Lambert Conformal Conic projection are swapped.  This doesn’t seem like much, but the transformation equations suggest that it may lead to sub-millimeter differences in the calculated y-coordinates.  For most applications, these differences are not worth worrying about.

However, if you do feel like worrying, it’s very easy to teach new projections to QGIS.  Copy the Proj4 definition from SpatialReference.org and paste it into the Settings > Custom CRS dialog.  Give it a name and hit the tiny save icon beneath the parameters box.  Now you can access your ESRI projection in QGIS.

Adding EPSG coordinate systems to ArcGIS is also possible, but requires a couple more clicks.  In ArcCatalog, right click a shapefile with the ESRI projection to be modified.  On the XY Coordinate System tab, click the Modify button.  Now you can give your projection a new name and swap the values for Standard_Parallel_1 and Standard_Parallel_2.  Click okay to save the file with the modified projection.  This returns you to the shapefile properties dialog, where you can click Save As to save the projection for use with other files.

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