# [Gdal-dev] USGS vs. ESRI vs. OGR

Richard Greenwood Rich at GreenwoodMap.com
Thu Dec 11 19:10:34 EST 2003

```At 01:38 PM 12/11/2003, you wrote:
>I followed your advise to compare to the USGS tool.  The conversion
>results are listed below.  As we can see, the ESRI and USGS tools match
>identically (to the precision of ESRI which was lower than the USGS
>solution).  There is a convergence parameter from the USGS calculation
>which is significantly larger than the error between OGR and ESRI/USGS.
>Being a novice at this reprojection business, I don't know if that is
>the bounds of the accuracy of the solution.  I would tend to believe not
>however, since a 40 minute minimum accuracy would be a multi-kilometer
>error in position.  On a hunch I looked at the two definitions used for
>WGS84 by both OGR and ESRI, and their coefficients match to reported
>accuracy.  I'll continue investigating this issue if no one has any
>ideas what could be causing it.
>
>UTM X   597196.62       Meters
>UTM Y   4116325.5       Meters
>
>Product Lat                     Lon                     Convergence
>Scale
>                 DD MM SS.sssss  DDD MM SS.sssss         DD MM SS.ss
>Unitless
>USGS            37 11 24.57282  085 54 17.71610         0 39 43.15
>9.99716E-001
>ESRI            37 11 24.57     085 54 17.72
>OGR             37 11 24.76           085 54 17.62
>
>Hank Grabowski
>hgrabowski at stk.com
>1-610-578-1000

The convergence angle is not an indication of error, it is the angular
difference between true (geodetic) north and grid north at a given point in
a projected coordinate system. Similarly, the scale is the ratio of the
difference between grid and ellipsoidal (sea level) lengths at a give point
within a projection.

WGS84 and NAD27 are datums, while Albers is a projection. Converting
between projections is rigorous i.e. a single formula can be applied at any
location in a projection to convert it between projections. Conversions
between datums are approximate. Conversions between datums are usually done
by interpolation. The datum shift will be different at different places on
the earth. So it is important that you do not attempt datum conversions
outside of the area of the datum (NAD27 is a localized datum, NAD83 and
WGS84 are not). Are your longitudes above west? And if so, shouldn't they
be negative?

Corpscon (which is based on Nadcon) is a widely accepted standard for
http://crunch.tec.army.mil/software/corpscon/corpscon.html

Richard W. Greenwood, PLS
Greenwood Mapping, Inc.
Rich <at> GreenwoodMap <dot> com
(307) 733-0203
http://www.GreenwoodMap.com

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