Using Map Projections and Datasets

For more information on mapping projections and in-depth discussions of algorithms and uses of the projections PV‑WAVE generates, refer to the following publications:

Map Projections Used by the U.S. Geological Survey, Geological Survey Bulletin 1532, John P. Snyder, Second Edition, 1983.

An Album of Map Projections, U.S. Geological Survey Professional Papers 1453, John P. Snyder and Philip M. Voxland, 1989.

Both are available from:

USGS ESIC: Open File Report Sales
Box 25286, Building 810
Denver Federal Center
Denver, CO USA 80225
Phone: (303) 236-7476
FAX: (303) 236-4031

A central problem facing cartographers for centuries was how to represent the features of a spherical globe on a flat map. The methods devised for “flattening” the globe onto a map are called map projections. PV‑WAVE can generate 17 different types of projections. It is also possible for you to design your own algorithms and use them in PV‑WAVE.

A map projection transforms spherical coordinates into two-dimensional X-Y coordinates. The spherical coordinates of the globe are defined by lines of longitude and latitude.

Longitude is the angle in degrees east or west of the prime meridian passing through Greenwich, England, and latitude is the angle in degrees north or south of the Equator.

Each different map projection preserves different characteristics of the globe it represents. The characteristics preserved by four important projection types are described below:

Equal Area Projection—Preserves the relative area of features at the expense of distortions in shape, angles, and scale. In an equal area projection, a coin placed on any part of the map will cover the same area.

Conformal Projection—Preserves the shape of small features correctly, but large feature are distorted. Most large scale maps use some type of conformal projection.

Equidistant Projection—Preserves the scale or measured distance between certain points and all other points on the map. This allows true distances to be measured using a ruler.

Azimuthal Projection—Preserves local direction, or the angle between one point and other points on the map.

Projections can exhibit one or more of the above properties, thus there are azimuthal equidistant and azimuthal equal-area projections. There are subclasses of these projections which preserve more specialized characteristics. On Mercator projections all rhumb lines (lines of constant direction) are shown as straight lines, and on Stereographic projections all small circles (e.g. lines of latitude) and great circles (intersection of a plane passing through the center of the sphere and the surface of the sphere) are shown as circles on the map.

In order to achieve some of the above properties, map projections are usually constructed in such a way that the surface of a sphere is “projected” or mapped to either a cylinder, cone, or plane (referred to as azimuthal). Thus the projection may refer to “Conic”, “Cylindrical”, or “Azimuthal” in its name to identify the construction method.

PV‑WAVE can generate the following map projections:

In PV‑WAVE a map dataset is a set of polylines (a series of connected points) or polygons (points which describe a filled area). These polylines or polygons can have a number of classification attributes associated with them which aid in selecting features to be plotted. These attributes allow the desired polylines or polygons for a map to be selected based on the area being mapped and the features you want to plot.

Two datasets are included with PV‑WAVE for creating world and US maps: The World Databank II dataset for global maps, and a dataset based on the USGS Digital Line Graph data for U.S. maps. In addition, a USGS database of U.S. map information is included with PV‑WAVE.

World Databank II is the default dataset used by the PV‑WAVE MAP procedure. The World Databank II dataset is a subset of a public domain dataset provided by the U.S. Department of Commerce, merged with updated country data from the National Imagery and Mapping Agency (NIMA). All of the attribute information from the original dataset is provided in PV‑WAVE, but the resolution has been reduced by sampling the polylines in order to make the dataset manageable both in terms of disk space and memory requirements.

The subsetted dataset contains approximately 300,000 points, which provides good resolution for most applications. The dataset consists of a series of polylines, and each polyline has attributes associated with it. You can subset a map by specifying these attributes with the Select keyword (to the MAP procedure). The attributes include coastlines/islands/lakes, rivers, international boundaries, and U.S. state boundaries.

The USGS Digital Line Graph Dataset is composed of polygons that draw U.S. states and counties. The polygons allow you to create either line maps or maps filled with color. The Select keyword (to the MAP procedure) lets you plot specific states and counties. This dataset can be selected by using the keyword Data = 'usgs_db' with the MAP procedure.

The USGS_NAMES function queries a built-in database of populated places in the United States. This database lets you find the longitude and latitude and for most cities and towns in the U.S. In addition, you can use the database to determine the FIPS codes for states and counties. See the PV‑WAVE Reference for details on this procedure.

The World Databank II and USGS Digital Line Graph datasets are provided with PV‑WAVE, as are procedures used to read them into PV‑WAVE. To read another dataset other than World Databank II and USGS Digital Line Graph Dataset, you must write a procedure tailored to read that dataset. For more information, see "Accessing Other Map Datasets".