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

[Grabowski, Hank]

I'm pulling the value for X and Y directly off of the ArcMap measuring
tool.  I zoomed into a point on a map that was drawn in the native
coordinate system of the shapefile (so there is no conversion) and then
entered those values into the website tool as you saw them.  To find the
WGS84 values I created a layer with the WGS84 coordinate system and then
added the data that had been converted by OGR into WGS84 and allowed
ESRI to auto-convert the data from the native shape file.  Again using
the measuring tool I got the exact measurement of the given point.  The
difference is quite visible without going into that much detail, but I
wanted to get quantitative differences to help diagnose the problem.  I
can try the same computation with the link you gave me in a previous
e-mail and move see what happens.  If you guys want I could post the
data, it's very small, on an FTP site for you guys to try and duplicate
the problem I'm seeing.  

Hank Grabowski
hgrabowski at stk.com
1-610-578-1000
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-----Original Message-----
From: Richard Greenwood [mailto:Rich at greenwoodmap.com] 
Sent: Thursday, December 11, 2003 7:11 PM
To: gdal-dev at remotesensing.org
Subject: Re: [Gdal-dev] USGS vs. ESRI vs. OGR




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 

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