Methods and apparatuses to generate and to access compressed and indexed
elevations of Digital Elevation Models. In one aspect of the invention, a
method to store elevation data includes: compressing elevation data of a
first portion of a Digital Elevation Model (DEM) to generate first
compressed elevation data; storing the first compressed elevation data in
a storage location pointed to by a first index; and storing the first
index. In another aspect of the invention, a method to retrieve elevation
data includes: locating a first compressed portion of a Digital Elevation
Model (DEM) using a first index; and decompressing the first compressed
portion to retrieve first elevation data for at least one sample point in
the Digital Elevation Model. The Digital Elevation Model has a plurality
of compressed portions which includes the first compressed portion; and
the first index points to a storage location where the first compressed
portion is stored.