Global Mapper recognizes many raster file formats, but can not always determine if the file represents elevation data or imagery. The following dialog will appear for some data when the type can not be determined, asking to specify the characteristics of the data set. This typically has to do with the bit depth and number of bands in the file.
If the raster represents elevation data or other numerical values that represent quantities, choose Yes to load the data an elevation grid. This will allow for changing the display using the elevation shader, viewing in 3D with pixel values representing heights, and running raster tools that work with elevation data, such as generating contours.
If the file is an image file, choose No to load the file as an image. Loading as an image file enables raster options such as Color/Contrast Adjustment.
Global Mapper can import images files that do not contain spatial reference information. This may include formats that do not support spatial reference information, or spatial data formats that are missing spatial reference information.
If there is a .prj , and world file or .gcp file with the same file name in the same folder as the raster, that information will be used to spatially locate the data for any supported raster format.
A world file or .gcp file will be used as control points to relate the raster pixels to a coordinate system. The .prj file will define the projection of that system. If only the control points are defined, there will be a prompt to confirm the coordinate system.
JPEG images may also contain EXIF data in the image metadata. Most modern digital cameras record EXIF data with the coordinates where the image was taken. Many image applications also provide options for geotagging , which includes this EXIF data in the image metadata. To control this behavior see Raster Formats Configuration
If there are no control points defined in the image header or included in a world file or .gcp file of the same file name, the following set of options will be available:
This option loads the pixel rows and columns as coordinates in the current projection. The image will show up at the origin (0,0) of the current projection.
Using the EXIF data (geotag) embedded in the image file, this will create a point feature at the specified location, with a special attribute (IMAGE_LINK) that links to the image.
The picture will display when selecting the point with the info tool. Hold down the CTRL key to instead view the point information. For more information about viewing the picture see Special Info Tool Behavior
With the digitizer tool, additional tools are available in the context menu to Place Picture image on map....
Check this option to maintain the setting for all files being loaded.
For more information on specifying the projection of loaded data see Loading Files with Unknown Projection