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High-Resolution Continent Outlines

IDL supports two different datasets that contain continent outlines and other geographical and political boundaries. The default data set is a low-resolution continental outline database that is automatically installed when you install IDL. The high-resolution database was adapted from the 1993 CIA World Map database by Thomas Oetli of the Swiss Meteorological Institute. The high-resolution outlines are found in an optional data set that may not have been installed when your copy of IDL was first installed.

To access the high-resolution data set, simply set the HIRES keyword when calling MAP_CONTINENTS with the COASTS,  COUNTRIES,  FILL_CONTINENTS, or RIVERS keywords. You can also get high-resolution continent boundaries by calling MAP_SET with the HIRES and CONTINENTS keywords set. See MAP_CONTINENTS for an example of using the high-resolution outlines.

Resolution of Map Databases

Data points in the CIA World Map database are approximately one kilometer apart. Note, however, that in the case of the coast and river databases, actual distances between the data points may be much smaller because of convolutions in the coastline or riverbed.

Data points in the low-resolution map database are either a subset of the high-resolution database (rivers and country boundaries) or are based on the continental map database used in previous versions of IDL (the file supmap.dat in the resource/maps subdirectory of the IDL distribution). Data points in the low-resolution database are approximately 10 kilometers apart.

Neither of the map databases is intended for high-precision work.

The following table compares the low-resolution and high-resolution map databases:

Table 9-1: Comparison of Low- and High-resolution Map Databases

Table 9-1: Comparison of Low- and High-resolution Map Databases
Coastlines, islands, and lakes (including continental outlines)
Data in file supmap.dat.
Entire CIA World Map
Continental polygons
Data extracted from supmap.dat.
Every 20th point of CIA World Map.
Every 250th point of the CIA World Map.
Entire CIA World Map.
National boundaries
Every 100th point of CIA World Map.
Entire CIA World Map.

  IDL Online Help (June 16, 2005)