[Gdal-dev] conversion from different AEA projections

[Dylan Beaudette]

Greetings!

I would first like to thank all those that have helped to make GDAL a reality 
- it is an extremely powerfull set of tools that make GIS with opensource 
software a possibility. 

I am having unexpected results when converting a raster dataset between two 
versions of the AEA projection. The resulting image is offset about 35km to 
the north of where it should be.

Here are the details:

1. the command i have been using to project the raster image is;
gdalwarp \
-t_srs '+proj=aea +x_0=0.0 +y_0=0 +lon_0=-96 +lat_0=40.0 +lat_1=20 +lat_2=60.0 
+datum=NAD83'  \
-s_srs '+proj=aea +x_0=0.0 +y_0=-4000000 +lon_0=-120.0 +lat_0=0.0 +lat_1=34.0 
+lat_2=45.5 +datum=NAD83'  \
-multi original.tiff test.tiff

This is essentially (attempting to) convert between 2 different AEA 
projections.

2. the output looks somthing like this:
http://169.237.35.250/~dylan/temp/offset.png

It is quite obvious in the image that the original.tiff (the landsat image 
behind the vector polygons) is offset mostly to the north...

3. here is what gdalinfo says about the input image:

The original image, original.tiff, has the following projection parameters:
 `gdalinfo original.tiff `:
---------------------------
GEOGCS["NAD83",
        DATUM["North_American_Datum_1983",
            SPHEROID["GRS 1980",6378137,298.2572221010042,
                AUTHORITY["EPSG","7019"]],
            AUTHORITY["EPSG","6269"]],
        PRIMEM["Greenwich",0],
        UNIT["degree",0.0174532925199433],
        AUTHORITY["EPSG","4269"]],
    PROJECTION["Albers_Conic_Equal_Area"],
    PARAMETER["standard_parallel_1",34],
    PARAMETER["standard_parallel_2",40.5],
    PARAMETER["latitude_of_center",0],
    PARAMETER["longitude_of_center",-120],
    PARAMETER["false_easting",0],
    PARAMETER["false_northing",-4000000],
    UNIT["metre",1,
        AUTHORITY["EPSG","9001"]]]
---------------------------

It might be useful to note that this data is coming from gis.ca.gov, formerly 
known as the TEAL data center. The source projection is what they call the 
"California Albers" projection. 

Also, I can get the raster image *close* to where it should be by using a 
-s_srs '+y_0=-3973800' instead of -s_srs '+y_0=-4000000' .. but there is 
still some residual distortion near the top of the image

Any ideas would be greatly appreciated!

Dylan Beaudette
Soil Science Graduate Group
University of California at Davis