Reproject high bit depth images

[jlertola]

My biggest wish is the ability to reproject high bit depth images. Most of the maps I make use elevation data to make a 3D map with terrain. All the elevation database I have, such as GTOPO, SRTM and NED contain 16 bit data. At the moment I have to degrade the data by converting it to a gray scale image so I can reproject the data with Geocart before I bring it into a 3d program.

Thanks,
-Joe Lertola

[daan]

Hello, Joe. Geocart reprojects in 16-bit/channel RGB. If it is a whole-world image in “geographic coordinates” projection, then just save the data as a 16-bit/channel TIFF file. You can use that as a database as-is because if the horizontal pixel count is twice the vertical, Geocart will assume it is intended to be a world database in “geographic coordinates” projection. If you have smaller regions than whole-world, then instead you will have to treat it as a general raster reprojection data base. Place the image into a new document and set up a skeletal map over it to describe the boundary and projection details. See the manual’s explanation of “Export Database”.

[jlertola]

Oh no. It is not working for me. I know what the problem is. My databases are 16 signed integers and rgb images are unsigned. When I make a database with my elevation data in one of the 16 bit rgb channels Geocart anti-alias the edge between positive and negative numbers which produces incorrect values. One solution that I can think of is that I could write a little program that can add a number to each value in the database to keep all the values positive and then subtract the number after that database has been reprojected.

-Joe

[daan]

Or, perhaps, you could use Photoshop to add a 50% constant gray value image to the channel you are using for elevation?

[jlertola]

Yes you are correct. I can make the adjustment in Photoshop. I need to add 50% gray to above sea level pixels and subtract 50% gray from below sea level pixels which I can do with Photoshop's Calculations... command. I am happy that I can now reproject these databases.

Thanks,

-Joe