The thematic maps we have discussed are based on vector layers. MapInfo Pro takes thematic mapping to a new level with a method that displays data as continuous color gradations across the map. This type of thematic mapping, known as grid mapping or surface theme mapping, is produced by an interpolating point data from the source table. MapInfo Pro generates a grid file from the data interpolation and displays it as a raster image in a Map window.
Grid theme maps are appropriate analytical tools in traditional GIS environments and other industries where the data points have measured values that reflect those locations. For example, use grid shading to illustrate temperature changes, snowfall amounts, or change in elevation.
The next figure shows the light to dark gradual transition across the United States, which represents low to high average annual temperatures. The continuous shading allows you to derive measurements at places other than where measurements were taken.
Figure: Grid Surface Thematic Map
See also:
About Grid Image Files
Grid mapping produces a file type called a grid image. When you create a grid thematic map using MapInfo Pro's grid handler, MapInfo Pro generates a default file name of tablename_fieldname with the extension .MIG (for example, USELEV_ELEVFEET.MIG). In the Create Thematic Map - Step 2 of 3 dialog box MapInfo Pro displays the path and the root file name, where you can change it as necessary.
MapInfo Pro contains one read/write grid handler and a Grid Engine tool kit that shows you how to create your own. Once you have written a grid engine, you place this file in your MapInfo Pro application directory and set the preference in the Map Window dialog box. Specifically, to set the default grid writeable handler, go to the Map window preferences and specify your grid handler file. The available handler formats depend on what grid handlers have been installed. Changing the default grid handler will change the default file extension of the grid file name.
Grid files are stored by default in the data directory specified as a preference. Grid files can be opened from the HOME tab. Click the Open command like other MapInfo Pro supported file types.
When you open a table that has a grid filename associated with it, MapInfo Pro searches for the file if it cannot find it immediately. For example, the search capability can simplify opening tables if your .TAB file refers to an image that resides on a DVD drive, and different letters are used to designate the DVD drive on different systems.
MapInfo Pro uses the following search order:
- Search for the file where specified by the FILE tag in the .TAB file.
- Search for the file in the same directory as the .TAB file.
- Search for the file in the Table Search path specified on the PRO tab, by clicking Options, and Directories.
MapInfo Pro either opens the table as though it found the file in the specified location, or it will report an error because the file could not be found in any location.