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I have two GeoPDF maps of the same area published a few years apart. There are only minor updates so virtually identical. However the newest map appears to be incorrectly georeferenced. I ascertained this by; loading the map in QGIS and checking the location of various features including the intersection of the UTM grid, extracting the UTM grid to JSON and looking at the grid node coordinates (should be even 1000 multiples), merging an adjoining map using gdal and noticing grid and features like contours are misaligned.

The incorrectly referenced map appears to be rotated (or skewed) slightly such that near the corners the georeferencing is out by about 25-30m.

Adjoining maps seem to be skewed the opposite direction so features crossing over the join are misaligned by about 50-60m

What can the GEOTransform parameters (Esp parameter [2] and [4]) tell me about how the map is rotated or skewed? I have the two maps parameters below. I would have thought that two maps of the same area with identical datum, projection and scale should have pretty much the same GEOtransform parameters. (Of course the Origin may be different as the map might not be centered the same) What I noted was the older correctly georeferenced map has parameter [2] and [4] that are not much different verses the newer map where they are quite different and this may indicate more rotation or skew?

Latest MAP

PROJCS["GDA_1994_MGA_Zone_56",
    GEOGCS["GCS_GDA_1994",
        DATUM["Geocentric_Datum_of_Australia_1994",
            SPHEROID["GRS_1980",6378137.0,298.257222101]],
        PRIMEM["Greenwich",0.0],
        UNIT["Degree",0.0174532925199433]],
    PROJECTION["Transverse_Mercator"],
    PARAMETER["False_Easting",500000.0],
    PARAMETER["False_Northing",10000000.0],
    PARAMETER["Central_Meridian",153.0],
    PARAMETER["Scale_Factor",0.9996],
    PARAMETER["Latitude_Of_Origin",0.0],
    UNIT["Meter",1.0]]
GeoTransform =
  221129.6669699433, 4.231395266408159, 0.1346913327803821
  6248054.609901944, 0.1185136891607636, -4.231917721984926

Older Version

PROJCS["GDA_1994_MGA_Zone_56",
    GEOGCS["GCS_GDA_1994",
        DATUM["Geocentric_Datum_of_Australia_1994",
            SPHEROID["GRS_1980",6378137.0,298.257222101]],
        PRIMEM["Greenwich",0.0],
        UNIT["Degree",0.0174532925199433]],
    PROJECTION["Transverse_Mercator"],
    PARAMETER["False_Easting",500000.0],
    PARAMETER["False_Northing",10000000.0],
    PARAMETER["Central_Meridian",153.0],
    PARAMETER["Scale_Factor",0.9996],
    PARAMETER["Latitude_Of_Origin",0.0],
    UNIT["Meter",1.0]]
GeoTransform =
  221155.2896939424, 4.233371207500782, 0.1186483868760786
  6248057.969519025, 0.1186525653949765, -4.234389541308537

You can check out the georeferencing of the actual map at: http://maps.six.nsw.gov.au/etopo/geopdf/25k/8930-3S%20KANANGRA.pdf Adjoining map: http://maps.six.nsw.gov.au/etopo/geopdf/25k/8930-3N%20JENOLAN.pdf The older (correct) map version is not publicly available

I would really like to know an easy way to correct the georeferencing. I am hoping by playing around with the geotransform I can do this. I can programatically get a number of reference points by extracting the UTM grid node coordinates and rounding them to the nearest even thousand to get the correct coordinates, but not sure the best way to re-reference the map. I would like to do this for 100 maps so want to do it programatically.

  • Look at the description of an Esri world file (an affine transformation) for an explanation of the parameters. 1&4 are x and y translations, 3&5 are skew/rotation values, 2&6 are x- and y-scale. – mkennedy Dec 19 '17 at 0:57
  • When I referred to parameters [2] and [4] above I was meaning starting at [0] so referring to same rotation/skew parameters as your 3 & 5. I am still not able to visualize what it means when these parameters are not equal. If they are the same but not zero I can see it is a simple rotation. – John Flint Dec 19 '17 at 8:55

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