Import DEM Data
miksmith
Global Mapper UserTrusted User
I've got some ERDAS Imagine files containing elevation data. When I import them with GM901 they are loaded as an image, not elevation data. How do I get them to load as elevation?
thanks
mike
thanks
mike
Comments
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We have recently made some changes to make users be prompted more often to specify whether Erdas Imagine files are imagery or elevation when it is ambiguous from the data layout in the file. I have placed a new build at http://www.globalmapper.com/global_mapper9.zip with the change for you to try. Simply download that file and extract the contents into your existing v9.xx installation folder to give it a try.
Let me know if I can be of further assistance.
Thanks,
Mike
Global Mapper Support
support@globalmapper.com -
Thanks for the update to GM for opening 8bit DEM files. I agree that unsigned 8-bit files are very unusual for storing elevation. This data comes from the Landmap project which is the UK and Ireland only. Values will have a maximum of ~4400 which means they should have been 16bit. Its possible that, for some reason, this data got saved as 8bit as the elevation in the area covered doesnt go above 256m.
Would it be possible to have an "Options" tab that gives the user the choice of specifying elevation at import for any dataset? -
Are you looking for a way to say that all ambiguous Erdas Imagine files are elevation or some way to specify that any loaded data set is elevation? The former would be possible (there is already a similar option for BIL import), but only a subset of raster formats support elevation interpretation. In general the interpretation of a data set is not ambiguous.
Thanks,
Mike
Global Mapper Support
support@globalmapper.com -
Hi Mike
If the interpretation of a raster dataset is not ambiguous thats fine. Im used to working with Imagine files hence the problem!
thanks
mike -
Mike,
I would suggest perhaps loading all of your Erdas files as once when you need to work with them, then select "Yes to All" when prompted for the type so that you only get asked once whether the data is elevation or raster. You could also save a workspace once the data is loaded and then just load that. One final option might be to use the File->Batch Convert menu command to batch convert your ambiguous Erdas files to a non-ambiguous format, like Global Mapper Grid, then just work with those non-ambiguous and much better compressed files.
Let me know if I can be of further assistance.
Thanks,
Mike
Global Mapper Support
support@globalmapper.com -
Hi Mike
Thanks for the thoughts he. Very helpful. The question of file formats is of interest for two reasons:
1. Speed of drawing: IMG files usually have an an associated RRD file which contains pyramid layers and dramatically speeds up redrawing the image. Whilst GMG files appear faster to redraw than IMG, it isnt as fast as in Imagine. Any thoughts on this at all?
2. Compression: I noticed that the GMG files are less than a third of the size of a ~170Mb IMG file. In the GMG export dialogue I noticed the option of lossy/lossless compression. Lossless is preferable with anything analytical so that is what I selected; what compression routine is this using out of interest?
cheers
mike -
Mike,
The GMG file internally uses a pyramid to speed up drawing. I'm guessing that Imagine still renders faster because its internal render engine is optimized to a particular type of drawing rather than being more generic for handling pretty much anything like the GM render engine.
The GMG format breaks the file into tiles internally and the tries numerous compression techniques on each tile. The one that results in the best compression is used. This is similar to the Arc Binary Grid format, except that GMG has a couple of extra compression techniques that it tries that the Arc format does not.
Let me know if I can be of further assistance.
Thanks,
Mike
Global Mapper Support
support@globalmapper.com
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