Global Mapper v25.0

Elevation exported to GeoTIFF as Pixel is Area is tagged as Pixel is Point

tjhb
tjhb Global Mapper UserTrusted User
edited May 2011 in Technical Support
Mike,

I'm enabling the configuration option "Terrain Export: specify bounds as Pixel Is Area", then exporting elevation tiles cropped to selected areas.

When the GeoTIFFs are loaded back into GM, the metadata shows the interpretation "Pixel is Point".

Would it be possible to have the current interpretation, "Pixel is Area" or (by default) "Pixel is Point" stored in the GeoTIFF metadata during export?

I am using 12.02 from the installer dated 12 March 2011.

Tim

Comments

  • global_mapper
    global_mapper Administrator
    edited May 2011
    Tim,

    That option just controls how the specified bounding coordinates are interpreted from the Export Bounds dialog. For export to a vertical GeoTIFF file those coordinates will still be converted to "pixel is point" coordinates, they will just be translated differently from what you entered depending on whether or not that option is checked. Typically vertical GeoTIFF files use "pixel is point" and imagery GeoTIFF files used "pixel is area" (Global Mapper supports reading them either way). Do you have a particular need for vertical GeoTIFF files that specify the coordinates as "pixel is area"?

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • tjhb
    tjhb Global Mapper User Trusted User
    edited May 2011
    Typically vertical GeoTIFF files use "pixel is point" and imagery GeoTIFF files used "pixel is area" (Global Mapper supports reading them either way). Do you have a particular need for vertical GeoTIFF files that specify the coordinates as "pixel is area"?

    I agree that that is typical (it's a topic we've discussed before) but yes we most often do need vertical GeoTIFF files that use the "pixel is area" model.

    There are two reasons for that. One is that for accurate cartographic workflows we need to have imagery and elevation data that have both identical geographic extents and identical pixel dimensions. The second reason is that Manifold System uses a single storage model, namely "pixel is area", for both types of raster. This has a certain purity to it, though it probably causes almost as much confusion as it saves. Analysis on a "pixel is point" model is still possible by using different maths. Strictly speaking, the unified model only affects storage.

    We can achieve our objectives in practice by using your "pixel is area" model when exporting elevation data. Manifold (and, just as importantly, Photoshop) will effectively see the result using the "pixel is area" model, and location will be exactly aligned with imagery data treated in the same way.

    I think I either disagree or misunderstand when you say—
    For export to a vertical GeoTIFF file ["pixel is area"] coordinates will still be converted to "pixel is point" coordinates, they will just be translated differently from what you entered depending on whether or not that option is checked.
    Elevation data exported with the "pixel is area" option enabled really has been (re)aligned to a grid whose centres are one-half-cell displaced from the centres of a "pixel is point" grid covering the same locations.

    It is disconcerting that elevation data exported by GM on the "pixel is area" model is nevertheless given the "pixel is point" attribute in GeoTIFF metadata. The (mis)classification is respected, correctly but to incorrect effect, when the data is read back in.

    Ideally (in my view), Global Mapper would treat "pixel is area" and "pixel is point" as equally valid storage models for gridded data. And it currently goes almost all of the way there. It can, as you say, read and correctly interpret both storage models. The recent "pixel is area" option in effect allows it to write to both. But GM will still treat its own "pixel is area" output as if it had been written as "pixel is point".

    This is not quite coherent in my view and can lead to interoperability errors (typically an apparent half-pixel shift) when working with other software.
  • global_mapper
    global_mapper Administrator
    edited May 2011
    Global Mapper does treat "pixel is area" and "pixel is point" as perfectly valid on import for GeoTIFF files when determining the display bounds for the layer. The coordinates are just shift by a half pixel spacing if necessary to generate the correct placement. In theory an application should check how the coordinates are stored in the file and adjust accordingly on import, but I can understand that some don't.

    I have updated the GeoTIFF elevation export to save the "pixel is area" representation of the corner coordinates rather than the "pixel is point" representation if you have selected that advanced option so that applications that only deal with "pixel is area" can get workable files. I have placed a new build at http://www.globalmapper.com/global_mapper12.zip with the change for you to try. Simply download that file and extract the contents into your existing v12.xx installation folder to give it a try. If you are using the 64-bit v12 version there is a new build at http://www.globalmapper.com/global_mapper12_64bit.zip .

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • tjhb
    tjhb Global Mapper User Trusted User
    edited May 2011
    Thank you Mike for making this small change. I will feel less puzzled now, and working with other users and with other software should be a little bit more bulletproof.