Geographic Informational Systems (GIS)

Spatial Information

Spatial polygon boundary layers are the “empty map” that must be created to visualize geographic space. This work is typically carried out by national governmental statistical or geoscience agencies. Governmental agencies create maps for different objectives. Differences are to be expected since each set of boundaries reflects local, regional, or national needs. Once created, these files are seldom edited for inaccuracies and may not reflect changes in those boundaries over time.

Line layers are used to display highways, streets, rail lines, rivers, and streams. Minor pathways and transportation linkages such as ferry crossings and bike trails are often missing.

Point layers are the most inconsistent information found in national data sets. What is regarded as an [Page 632]important landmark varies greatly. Some countries capture schools, hospitals, and civic buildings. Still others identify water wells and every residence. The accuracy of point data has improved with the use of global positioning systems (GPS); however, the completeness of point layers remains an issue.

A distinctive type of layer data is raster data. Elevation and temperature are examples of rasterized data. Raster data layers, while precise, are not necessarily uniform since the cell units to create raster layers can differ among data sets.

Data Sets

Most uses of GIS require tabular data that match the spatial layers. Problems may arise when the data are unavailable, incomplete, or do not match the boundary layers. Often, smaller spatial boundaries can be used as building blocks to approximate the available information, and estimations can be used for incomplete data.

Data Display

GIS displays information based on a projection method—a mathematical means of transferring information from a three-dimensional curved surface to a two-dimensional medium like paper or a computer screen. The choice of projection method has global ramifications. All projection systems create some type of distortion. For example, a projection that accurately represents the shapes of the continents will distort their relative sizes.

...