Digital Raster Graphic (DRG) or "raster" for short, is one of the features of GIS Pro. Technically speaking the basemap qualifies as raster data but for the purposes of this documentation it refers to bitmap images that are drawn over the top of the map. Currently the most popular bitmap image formats are supported, namely PNG, JPEG, BMP, and TIFF.
In order to properly draw raster overlays they must be geolocated. This is done upon import and remains with the image in the form of three or more tie points that link specific points on the image to points on the map. There are currently four ways to geolocate an image in GIS Pro:
Worldfile - A world file is a plain text file that accompanies an image and is named by a naming convention to associate it to the image. There are two naming conventions that are supported: 1) append a "w" to the image file's extension (example: house.jpg+house.jpgw), and 2) replace the second letter of the image file's extension with a "w" (example: house.jpg+house.jwg). The latter is preferred just because it appears to be more common and so it is used when exporting files with the Worldfile option. The Worldfile import method will only work when importing from iTunes as it requires two files (as opposed to opening an image from email). Opening a .zip file from email may work but this has not been verified.
KMZ File - The KML specification (KMZ is just a zipped KML) declares a "GroundOverlay" feature that accomplishes the task of drawing an image over the map. To position the image, KML defines a "LatLonBox" node which specifies the extreme coordinates of north, east, south, and west as well as a rotation in degrees. Google Earth's extension to KML offers a "LatLonQuad" specifies the four corner points of the image in latitude and longitude. This offers greater flexibility because it enables skew of the image, which is sometimes necessary to get it just right. GIS Pro supports both but prefers the latter and uses it when exporting. This is probably important to note because old versions of Google Earth will not support it.
GeoTIFF - GeoTIFF is an extension to the TIFF standard that enables geographic information to be embedded into the metadata tags of the TIFF file. This is handy because the file can be transported by itself and still retain its geospatial information. GIS Pro can read GeoTIFF but only supports the most common formats, which are UTM (with the zone representing the latitude band, not the hemisphere) and latitude and longitude. The default datum is WGS84 but datums NAD27 and NAD83 are also supported.
Manually - When all else fails a raster can be placed manually. This is not so bad, in fact, we have even been able to improve the positioning of many public raster examples! To begin, the process is done with three or more reference points. This enables scaling, rotation, and skew--something not all formats offer. Three points is sufficient to constrain the image and any more over constrains it. GIS Pro allows this and deals with the discrepancy using a least-squares of the error approach. In the future we may consider actually morphing the image (or "reprojecting" you might say), however, the examples we have worked with seem to work well enough without morphing.
When a raster image is imported without any geospatial information you will be presented with a dialog announcing the process.