A map is a graphic representation of spatial relationships on the Earth or other planets. Maps can take on many forms and sizes, depending on the intended use of the map. Maps no longer refer only to hardcopy output. In this manual, the maps discussed begin as digital files and may be printed later as GeoPDFs, slides, word processing documents, or hardcopy.
Some of the different types of maps are defined in the following table.
A map that shows the prevailing direction that a slope faces at each pixel. Aspect maps are often color-coded to show the eight major compass directions, or any of 360 degrees.
A map portraying background reference information onto which other information is placed. Base maps usually show the location and extent of natural Earth surface features and permanent human-made objects. Raster imagery, orthophotos, and orthoimages are often used as base maps.
A map portraying the shape of a water body or reservoir using isobaths (depth contours).
A map showing the boundaries of the subdivisions of land for purposes of describing and recording ownership or taxation.
A map portraying properties of a surface using area symbols. Area symbols usually represent categorized classes of the mapped phenomenon.
A map on which the combined information from different thematic maps is presented.
A map in which lines are used to connect points of equal elevation. Lines are often spaced in increments of ten or twenty feet or meters.
A map created by altering, combining, or analyzing other maps.
A reference map that outlines the mapped area, identifies all of the component maps for the area if several map sheets are required, and identifies all adjacent map sheets.
A map that is an enlargement of some congested area of a smaller scale map, and that is usually placed on the same sheet with the smaller scale main map.
A map that uses isorithms (lines connecting points of the same value for any of the characteristics used in the representation of surfaces) to represent a statistical surface. Also called an isometric map.
A map on which isopleths (lines representing quantities that cannot exist at a point, such as population density) are used to represent some selected quantity.
A map representing morphological features of the Earth’s surface.
A map showing the limits of a specific set of mapping entities, such as counties, NTS quads, and so forth. Outline maps usually contain a very small number of details over the desired boundaries with their descriptive codes.
A map showing only the horizontal position of geographic objects, without topographic features or elevation contours.
Any map that appears to be, or is, three-dimensional. Also called a shaded relief map.
A map that shows changes in elevation over distance. Slope maps are usually color-coded according to the steepness of the terrain at each pixel.
A map illustrating the class characterizations of a particular spatial variable (for example, soils, land cover, hydrology, and so forth)
A map depicting terrain relief.
A map showing only those areas visible (or invisible) from a specified point or points. Also called a line-of-sight map or a visibility map.
In ERDAS IMAGINE, maps are stored as a map file having a .map extension.
Thematic maps comprise a large portion of the maps that many organizations create. For this reason, this map type is explored in more detail.
Thematic maps may be subdivided into two groups:
A qualitative map shows the spatial distribution or location of a kind of nominal data. For example, a map showing corn fields in the United States would be a qualitative map. It would not show how much corn is produced in each location, or production relative to the other areas.
A quantitative map displays the spatial aspects of numerical data. A map showing corn production (volume) in each area would be a quantitative map. Quantitative maps show ordinal (less than/greater than) and interval/ratio (difference) scale data (Dent, 1985).
You can create thematic data layers from continuous data (aerial photography and satellite images) using the ERDAS IMAGINE classification capabilities. See Classification for more information.
Thematic maps should include a base of information so that the reader can easily relate the thematic data to the real world. This base may be as simple as an outline of counties, states, or countries, to something more complex, such as an aerial photograph or satellite image. In the past, it was difficult and expensive to produce maps that included both thematic and continuous data, but technological advances have made this easy.
For example, in a thematic map showing flood plains in the Mississippi River valley, you could overlay the thematic data onto a line coverage of state borders or a satellite image of the area. The satellite image can provide more detail about the areas bordering the flood plains. This may be valuable information when planning emergency response and resource management efforts for the area. Satellite images can also provide very current information about an area, and can assist you in assessing the accuracy of a thematic image.
In ERDAS IMAGINE, you can include multiple layers in a single map composition. See Map Composition for more information about creating maps.
Colors used in thematic maps may or may not have anything to do with the class or category of information shown. Cartographers usually try to use a color scheme that highlights the primary purpose of the map. The map reader’s perception of colors also plays an important role. Most people are more sensitive to red, followed by green, yellow, blue, and purple. Although color selection is left entirely up to the map designer, some guidelines have been established (Robinson and Sale, 1969).
- When mapping interval or ordinal data, the higher ranks and greater amounts are generally represented by darker colors.
- Use blues for water.
- When mapping elevation data, start with blues for water, greens in the lowlands, ranging up through yellows and browns to reds in the higher elevations. This progression should not be used for series other than elevation.
- In temperature mapping, use red, orange, and yellow for warm temperatures and blue, green, and gray for cool temperatures.
- In land cover mapping, use yellows and tans for dryness and sparse vegetation and greens for lush vegetation.
- Use browns for land forms.
In ERDAS IMAGINE, use Show Attributes option to select and modify class colors.