# [GRASS5] m.proj Difference?

Jeshua Lacock jeshua at sierramaps.com
Fri Jun 7 04:58:32 EDT 2002

```Greetings all,

I am trying convert some Albers Equal Area coordinates to UTM
coordinates, as I did with Grass Beta 11 and perhaps with earlier builds
(beta 8?).

It seems as I am doing everything the same way I did with Beta 11, and
yet I am getting slightly different results with Pre 4 then I did with
Beta 11.

I entered all of the same projection values as I did before, and when I
attempt to convert the same values I converted with Beta 11, I get
results in the Y within 0.7 meters, and the X is off by about 510
meters. I get similar results with adjacent coordinates being off by a
slight amount positively or negatively based on the direction traversed
from the first test point.

All of the coordinates that I converted with Beta 11 are on the money,
and I need to convert some more because of a data loss.

It seems like it must be a datum shift or some slight difference in the
ellipsoid parameters.

Following are the projection parameters I am using:

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

The values that Beta 11 correctly converted from the same AEA coords
above are:

289729.04706660  4181213.82628647
278734.48157092  4181410.37628647

The values that Beta 11 converted are exactly aligned with other outside
data sources.

Note that I used m.proj in beta 11, entered the same values in Pre 4
with m.proj to get the syntax for the proj2 call above. In Pre 4, both
proj and proj2 give me the values which are slightly off from old
successful conversions.

Am I just missing something? I greatly appreciate any help.

Here are the parameters I originally used in beta 11 (and in my Pre 4
attempts):

>                    Coordinate Conversions
>
>
>     1- Conversion       2- Input/Output Selection
>     3- Quit
>
>     Enter your selection  : 1
>
>
> Initializing INPUT PROJECTION:
>
> Enter 'list' for the list of available projections
> >aea
>
> Initializing INPUT projection ELLIPSOID:
> Enter 'list' for the list of available ellipsoids
> >clark66
>
>   Enter INPUT Central Parallel [lat_0] (23N) : 0
>
>   Enter INPUT First Standard Parallel [lat_1] (29:30N) : 34.0
>
>   Enter INPUT Second Standard Parallel [lat_2] (45:30N) : 40.5
>
>   Enter INPUT Central Meridian [lon_0] (96W) : -120
>
>     Enter INPUT False Easting [x_0] (0.000000) :
>
>     Enter INPUT False Northing [y_0] (0.000000) : -4000000.0
> Enter plural for INPUT units [meters]:
>
> Parameters used in INPUT projection:
>          +proj=aea +a=6378206.4000000004 +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
>
>
>
> Initializing OUTPUT PROJECTION:
>
> Enter 'list' for the list of available projections
> >utm
>
> Initializing OUTPUT projection ELLIPSOID:
> Enter 'list' for the list of available ellipsoids
> >clark66
>
>     Enter OUTPUT Projection Zone [zone] : 11
>
> OUTPUT Projection: Would you like to use South Hemisphere (y/[n]) ?
> Enter plural for OUTPUT units [meters]:
>
> Parameters used in OUTPUT projection:
>          +proj=utm +a=6378206.4000000004 +es=0.0067686580 +zone=11
> +unfact=1.0000000000
>
>                    Coordinate Conversions
>
>                  Coord->Lat/Long Conversion

With thanks,

Jeshua Lacock __________________________
Programmer/Owner            Phone:   760.935.4736
http://OpenOSX.com           Fax:        760.935.4845
-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_

```