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Cheat Sheet

GIS For Dummies

From GIS For Dummies by Michael N. DeMers

A GIS (geographical information system) is a fun and functional piece of equipment that offers maps, and so much more! You can analyze terrain and compare maps, keeping in mind the fact that the map you see is basically a model of the terrain A grid-based GIS offers some algebraic functions to help you fine-tune a search, and every GIS provides a variety of outputs from maps to charts to 3D diagrams.

What You Can Do with GIS

With GIS (geographic information system) you can do all sorts of geography-related stuff — find places, of course, but also find the best place to locate your business, among other things. The following list summarizes some of the tasks you can accomplish with GIS:

  • Find geographic features. You can search a GIS database to find point, line, area, and surface features by their descriptions or measurements.

  • Measure geographic features. You can measure lengths, widths, areas, and volumes, and compare sizes from one feature to another.

  • Characterize distributions. You can group geographic features and define their distributions based on how much space they use, how close they are to each other, and where they are relative to other features.

  • Summarize geographic data. You can calculate all sorts of statistics on your geographic features from the simplest descriptive statistics (for example, mean, median, and mode) to very complex spatial statistics.

  • Work with networks. You can find routes based on time, distance, or other factors. You can route buses to reach the maximum number of people and use this population density information to locate stores near your customers.

  • Compare map layers. You can compare the locations of features from one map layer (or theme) to another. This powerful feature helps you overlay the layers, and shows you the relative location of features from one layer to another.

  • Perform surface analysis. You can work on the many surfaces available in GIS and use mathematical methods (such as interpolation) to find missing values and perform other analyses.

Grid-Based GIS Map Functions

If your GIS (geographic information system) is grid-based, you have access to some cool, algebra-based functions. The following table shows the functions, where they work, and what you can do with each:

Function Type Where It Operates What It's Used For
Local On individual grid cells To change cell values based on user definition or the value of corresponding grid cells on other layers.
Focal On a specifically targeted grid cell To return a value (such as an average) based on the values of neighboring grid cells
Zonal On grid cells in specifically identified regions To calculate values based on analysis of specified regions that are not necessarily connected
Block On square blocks of grid cells To return a value for the identified block (for example, a 4 x 4 block of cells) on an output grid
Global On the entire grid To highlight hard-to-find features and spot general trends by moving through the entire grid
Specialty On specified grid cells To perform high-end statistical analysis or create models for moving surfaces (such as water or pollution)

GIS Map Characteristics to Keep in Mind

Your GIS (geographic information system) is very handy and a great navigation tool, but what you see on the GIS screen isn't necessarily what the actual terrain looks like. As you use your GIS, remember the following facts:

Map Characteristic What It Means
Maps are models — not miniatures Maps generalize geographic features by using symbols so that all features will fit the specified output size.
Map scale has a huge impact on GIS analysis Small-scale maps cover large areas with little detail, and large-scale maps cover small areas with lots of detail.
Maps are a flat model of a spherical earth Maps use projections to compensate for the flat versus spherical issue, and each projection has its own type and amount of distortion.
Maps have a reference grid, or coordinate system The reference grid helps you navigate the map and links the spherical earth to the map projection.
Maps have a reference starting point, or datum. Datums are based on a model of the Earth called a reference ellipsoid and enable all the various projections in a GIS work together to give an accurate picture of the Earth.

Types of GIS Output

You know that your GIS (geographic information system) provides maps — that's its basic function and probably the reason you bought it. But a GIS offers more than maps, and the following list includes other outputs:

  • Maps: Everyone recognizes this most common output from a GIS.

  • Cartograms: These special maps that distort geographic features based on their output values rather than their size.

  • Charts: GIS can produce pie charts, histograms (bar charts), line charts, and even pictures in addition to maps.

  • Directions: Another common output, directions show you how to get from one place to another.

  • Customer lists: Business GIS applications often produce customer lists, sometimes with printed mailing labels.

  • 3D diagrams and movies: These forms of GIS output help you see the results of your work realistically and dramatically.

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