This guide covers both GIS applications and concepts. The approach is operation oriented, making it necessary to present the same concept from different perspectives when the various GIS operations related to the concept are being introduced. In this chapter, we consolidate and supplement the concepts we have covered.

Coordinate System

A unique property of geographic features is that they are positioned in a coordinate system on which geometrical calculations are based. The dimensionality of a coordinate system indicates the number of coordinates we use to uniquely locate a point. A two-dimensional system uses two coordinates.

Geographic coordinates in degrees of latitude and longitude are based on ellipsoidal reference surfaces, and there are a number of these. Different ellipsoids are used to map different parts of the earth, and they are modified as we acquire more accurate measurements of the earth's shape.

Curved surfaces are not very convenient for display and for making measurements. The grid of latitudes and longitudes on an ellipsoidal surface form a network of curvilinear lines, making measurements, such as distance, angle, and area, more difficult than corresponding measurements on a flat surface. Hence the need to transform the earth's surface onto a flat map.

Please also refer to chapters on Geographic and Ground Coordinates, Data Conversion and Integration, Transformations.

Map Projection

A map projection transforms an ellipsoidal earth to a flat map, always causing some geometric distortions. Different map projections were developed to minimize one kind of distortion or another.

After being transformed onto a flat surface, it is then possible to use a rectangular grid to locate points and perform other geometrical measurements on the earth's surface. The units of measurement on such a grid are usually based on metres or feet on the ground, thus the term ground coordinate system.

Maps produced by different projections would not agree with each other in two aspects. First, they are distorted differently. Second, the ground coordinates of the same point from two different projections seldom agree because of the different rectangular grid being used.

Ground coordinates from any map projection could be converted back to geographic coordinates, and in theory the geographic coordinates of the same point from different map projections should agree. Discrepancies could be caused by computational errors and the different reference surfaces being used.

Please also refer to chapters on Geographic and Ground Coordinates, Coordinate Systems and Distance Measurement, Coordinate System and Area Measurement, Data Conversion and Integration.

Scale

A GIS database, unlike a conventional map, does not have a scale in the usual sense. Scale is an important characteristic of maps because they are rarely as large as the area they cover. In a digital database only coordinates are stored and there is no physical paper involved. Consequently the units of measurement for the coordinates can be made to be one-to-one, or in other words, scaleless. However, geographic data are often collected from documents such as maps and aerial photographs that do have scales. A scale indicator carried in the database helps to convey a sense of accuracy and level of detail.

Please also refer to chapters on Data Conversion and Integration.