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

[Richard Greenwood]

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 
projection and datum conversion and can be freely downloaded at: 
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