I work with navigational charts, which by industrial standard are made in Mercator projection. Each chart has individual latitude of true scale depending on the geographical area covered. Therefore I constantly need to use GeoTIFFs which are in Mercator projection(s).
Problem is that it seems that for some reason ArcGIS does not follow the GeoTIFF specification on this issue. If I read valid GeoTIFF made by another software, ArcGIS Desktop doesn't recognize the coordinate system info correctly. If I then (re)assign the coordinate system in ArcCatalog other softwares are not anymore able to align the data correctly.
Example: A navigational chart with 63d58m as latitude of true scale, GCS=ETRS89 (EPSG::4258)
Latitude of origin: 0 Longitude of origin: 0 False northing: 0 False easting: 0 Scale factor at origin: 0.440084879 (value which gives scale=1 at 63d58m)
If the GeoTIFF specification for CT_Mercator is followed, GeoTIFF tags would look like this: Keyed_Information: GTModelTypeGeoKey (Short,1): ModelTypeProjected GTRasterTypeGeoKey (Short,1): RasterPixelIsArea GeographicTypeGeoKey (Short,1): GCS_EUREF89 ProjectedCSTypeGeoKey (Short,1): User-Defined ProjectionGeoKey (Short,1): User-Defined ProjCoordTransGeoKey (Short,1): CT_Mercator ProjNatOriginLongGeoKey (Double,1): 0 ProjNatOriginLatGeoKey (Double,1): 0 ProjScaleAtNatOriginGeoKey (Double,1): 0.440084879 ProjFalseEastingGeoKey (Double,1): 0 ProjFalseNorthingGeoKey (Double,1): 0 ProjLinearUnitsGeoKey (Short,1): Linear_Meter End_Of_Keys.
Unfortunately ArcGIS does not use the scale information at all. It projects the GeoTIFF as having scale=1 at the Equator. The Spatial Reference information at Layer Properties looks like this:
As result the data is shown around latitude 34.75 instead of the correct area aroung 64. Of course the longitude is affected also, its around 10 instead of 23.
On the other hand if I assign the raster with following custom coordinate system information in ArcGIS, I get the data aligned correctly - but only in ArcGIS.
Projection: Mercator Linear unit: Meter (1.000000) Angular unit: Degree (0.01745.....) false_easting: 0 false_northing: 0 central_meridian: 0 standard_parallel: 63.96666666666667 Datum: D_ETRS_1989
When we look at the GeoTIFF keys it seems that ArcGIS would use latitude of true scale as origin which it actually does not do since the projected coordinate values are not 0 at that latitude. In other words ArcGIS writes Mercator (2SP) parameters in GeoTIFF's Mercator (1SP) fields. That causes major misalignment in softwares which interpret the GeoTIFF headers following the specification. My data ends up in latitude around 75.
Any suggestions how I can end up with a solution where I can read same true Mercator GeoTIFF-file in ArcGIS and other software (Geomedia for example) without changing the coordinate system metadata every time? Is the only feasible solution to create two versions every time when new GeoTIFF is created?
I want to correct one detail in my original post. I wrote that...
When we look at the GeoTIFF keys it seems that ArcGIS would use latitude of true scale as origin which it actually does not do since the projected coordinate values are not 0 at that latitude. In other words ArcGIS writes Mercator (2SP) parameters in GeoTIFF's Mercator (1SP) fields. That causes major misalignment in softwares which interpret the GeoTIFF headers following the specification. My data ends up in latitude around 75.
Strictly speaking behavior with coordinate values 0 at the latitude other than the Equator would not be Mercator (2SP)/EPSG::9805 but Mercator (Variant C).
