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Mapping Data

Maps are the core of geospatial analysis. Data stored in a map has a location on the Earth and a spatial relationship to other data in the map. The relationship is more than just "next to". On a map we can know exactly what direction and how far away. This is what makes maps useful and this section is designed to make using digital maps easier.

To put your data into a map you must know, measure, or otherwise obtain the location of each data value. The most common system for describing locations on the Earth (and other planets, too) is latitude and longitude. These spherical coordinates can be transformed, via a projection, into rectangular map coordinates (x,y). The datum value is then stored at the map coordinate location. To retrieve the geographic location of the data this process can be reversed. The rectangular map coordinates are obtained and then the inverse transformation gives the latitude and longitude.

This section has general information describing maps and how to use grids to store data, as well as map transformation code and examples of its use.

Mapx: A mapping and gridding transformations library in C and Java. Mapx implements functions for converting latitude and longitude to x and y and back again. The C version of Mapx currently supports 11 common projections, and the Java version supports 9, in both spherical and ellipsoid forms when appropriate. Both are object oriented so the addition of new map projections takes minimal effort.

JAZ: A JavaScript search interface widget for selecting an area of interest. (An older Java version of JAZ is no longer supported.)

AS2GT : The AMSR Swath to Grid Toolkit. AS2GT is a good example of using the C version of Mapx. Take a look at the file gsgrid.c.

MS2GT: The MODIS Swath to Grid Toolbox. MS2GT is another example of using the C version of Mapx. See the file ll2cr.c.

Points, Pixels, Grids, and Cells : A Mapping and Gridding Primer. This is a tech memo (part of the Mapx documentation) that gives a brief introduction to mapping, gridding, and the associated terminology used in the Mapx library.

EASE-Grid : Equal-Area Scalable Earth Grid. EASE-Grid is a good example of storing data in a mapped grid. Originally designed for a time series of passive microwave brightness temperatures from SMMR and SSM/I, EASE-Grid has gained wider usage and is now one of the EOS Core System standard projections.

Fortran I/O : It can be difficult to use Fortran to read gridded binary data. This tech memo describes tips and tricks for Fortran programmers who want to use grids.