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I am working to replicate an ArcInfo AML processing script in Python and am running into issues precisely replicating the raster output. In brief, the AML script reprojects input data from a Lambert projection on a sphere to another projection. However, when the Python output is compared to the AML output, significant differences show up.

With some testing, I believe I have tracked down the issue replicating the projection definition in the AML processing. In the AML, the projection is defined as:

projection lambert
spheroid sphere
units meters
parameters
33 00 00 /* 1st std parallel
45 00 00 /* 2nd std parallel
-97 00 00 /* central meridian
40 00 00 /* latitude of origin
0 /* false easting
0 /* false northing

In Python, I am using:

PROJCS["Lambert_Conformal_Conic",
    GEOGCS["unnamed ellipse",
        DATUM["unknown",
            SPHEROID["unnamed",6370000,0]],
       PRIMEM["Greenwich",0],
        UNIT["degree",0.0174532925199433]],
    PROJECTION["Lambert_Conformal_Conic_2SP"],
    PARAMETER["standard_parallel_1",33],
    PARAMETER["standard_parallel_2",45],
    PARAMETER["latitude_of_origin",40],
    PARAMETER["central_meridian",-97],
    PARAMETER["false_easting",0],
    PARAMETER["false_northing",0],
        UNIT["Meter",1.0]]

The only apparent differences could be the radius of the sphere or the degree definition, which are explicitly stated in the Python projection definition but appear to use a default in the AML. Does anyone know what these assumed default values would be in ArcInfo AML or have any other suggestion for addressing this issue?

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  • Please don't include pictures of text data. Instead, include the text itself.
    – Vince
    Oct 20 '20 at 13:40
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    Edited to replace image of AML projection with text.
    – nrp1000
    Oct 20 '20 at 15:03
  • 2
    "sphere" in ArcInfo CRS format is an approximate authalic sphere radius based on NAD27, so use 6370997.0. I guess I should make that an answer.
    – mkennedy
    Oct 23 '20 at 23:07
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The "sphere" keyword used with SPHEROID represents an approximate authalic radius (sphere) based on the Clarke 1866 ellipsoid. The value is 6370997.0 meters.

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  • Thanks! I am testing and will report back on whether this solves the problem using the new datum definition: GEOGCS["GCS_Sphere_Clarke_1866_Authalic", DATUM["D_Sphere_Clarke_1866_Authalic", SPHEROID["Sphere_Clarke_1866_Authalic",6370997.0,0.0]], PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]]. Either way, this answers the question asked.
    – nrp1000
    Oct 26 '20 at 2:13

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