[GRASS5] m.proj Difference?

Morten Hulden morten at ngb.se
Fri Jun 7 12:57:53 EDT 2002

On Fri, 7 Jun 2002, Jeshua Lacock wrote:

> m.proj2 inproj="+proj=aea +a=6378206.4 +es=0.0067686580 
> +lat_0=0.0000000000 +lat_1=34.0000000000 +lat_2=40.5000000000 
> +lon_0=-120.0000000000 +x_0=0.0000000000 +y_0=-4000000.0000000000 
> +unfact=1.0000000000" outproj="+proj=utm +a=6378206.4 +es=0.0067686580 
> +zone=11 +unfact=1.0000000000" input=test.coord output=results.coord
> These are the AEA coordinates (in test.coord):
> 54460.32787477583042 -28942.57618516702496
> 43488.83566543841880 -29097.67753145935785
> Which with Pre4 is converts to:
> 290240.740383  4181213.128503
> 279250.751623  4181409.678191

Tested with external proj and get exactly the same results. This means 
m.proj is performing correctly, and the difference must come from 
different a and es values in your previous (beta 11) conversions.

> The values that Beta 11 correctly converted from the same AEA coords 
> above are:
> 289729.04706660  4181213.82628647
> 278734.48157092  4181410.37628647

m.proj reads the a and es values from GRASS's ellipse table 
(etc/ellipse.table). The current ellipse.table does not have an es value 
for clark66, but an f-value instead, from which es is calculated. An older 
version of ellipse.table might have had hardcoded es-values and this may 
account for the differences you see.

Anybody able to say if ellipse.table has changed between beta11 and 
pre4? Is the f-value given for clark66 in current ellipse.table correct? 
Is the conversion routine f->es correct?

Morten Hulden

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