ARC/Info Geographic Information Systems

GIS = DATA + TECHNOLOGY + PEOPLE

 Introduction to Geographic Information Systems
Written by George Lienkaemper

Geographic Information Systems are computer applications that enter, store, retrieve, and analyze spatial information.
 
Data Capture
Data Processing
Data Output
Digitizing - electronic signals that depict map features Menus make the commands easier to use, but they don't outline the process GIS should produce new information from analytical process
Data from other sources - no control on quality GIS software is powerful because of its flexibility New maps can provide a different way to view data
Text files - existing data GIS are poor data base management tools GIS output can be ported to other software for additional processing
Interactive input - heads up digitizing Can work with vairous data set sizes Versatile

Vector maps are combinations of point, linear, and areal features along with concomitant annotation (points, lines, and polygons.

Features in a GIS are defined by location in space (registration), non-spatial information (attributes), and the relationships among the features on a map (topology).

Data are structured into broad categories called themes, which are composed of several data layers, represented on maps as features - points, lines, and polygons.

Raster maps represent data as grid cells. Raster data can be continuous, discrete, or classified. Individual cells are registered in space, and have specified size or resolution.

Images are composed of cells called pixels (picture elements) which contain values that represent measures of reflected radiation from the earth's surface. Images may contain data from numerous spectral bands.

GIS Functions

Editing Functions

A GIS has tools with which you can pan and zoom, add and delete lines, points and annotation. Some systems have more of this capability than others, but all systems have a digitizing module for data capture. Reclassification Functions Features can be selected based on attribute characteristics and reclassified to create a new map. Very complex queries can be performed. Some systems attach to external relational or object-oriented databases, which allow for calculating new variables and reclassifying based on those values. Overlay Functions One of the powerful analytical tools of a GIS is the process of combining information by overlaying two data layers. All sorts of boolean logic and mathematical and trigonometric functions can be applied to these overlays. The wonder of it all is that it's a complete topological overlay, so that the new information takes on all the characteristics of the old. Proximity Functions This group of tools includes buffering to a specified distance around map features, searching for occurrences within a range, nearest neighbor searches, clumping functions, and more! DEM Functions Many of these tools probably belong in one of the above categories, but what they have in common is that they all act on the digital equivalent a topographic map - the digital elevation model of DEM. A GIS will calculate slope, aspect, and elevation zones; as well as generate viewsheds or watershed boundaries, create contour lines and perform numerous other valuable feats. Data models encourage question-driven GIS analysis, identify what data are required, and provide a pathway through the wilderness.

Data dictionaries provide appropriate documentation for digital data.

GIS IS ONLY A TOOL!

 Arc/Info Coverages

Vector maps in Arc/Info are called coverages. A coverage is comprised of a feature type (points, line, or polygons) and numerous associated files. In fact, a coverage becomes a further sub-directory that contains a number of files related to registration, attributes, and topology of digital data depicted in the map. We cannot access all these files, but three that we can are very important.

.BND - this file contains the coordinates of that mark the full extent of the coverage (including any blank space). Whenever we use mapextent (set a window) in Arc/Info we access the .BND file.

.TIC - this file contains the coordinates or the map registration (or tic) marks that orient the coverage in space. At least four tic marks must be included when any map is digitized using Arc/Info. This file is very important when registering two coverages or when changing map projections.

Feature Attribute Tables

.PAT - this file contains the attributes associated with either Polygon or Point features (polygon or point attribute table). Initially .PAT files contain the area, perimeter, an internal ID, and a User ID for each point or polygon; but often the file is expanded to include other attributes.

.AAT - This file contain attributes associated with linear or arc features (arc attribute table). Initially .AAT files contain 'to' and 'from' node numbers, right and left polygon numbers, length, internal and User IDs for each arc feature. This file is also often expanded to include other attributes.

 Grids and Lattices Raster or cell data in Arc/Info are organized into grids or lattices. Lattices contain continuous data (often floating point values) and grids depict discrete data. Like coverages, grids and lattices are subdirectories containing several files that describe them topologically. Three files associated with grids can be accessed.

.BND - this file is identical to those associated with coverages.

.VAT - this file lists the frequency of the values in a grid (value attribute table). It can be expanded to include other attributes.

.STA - this file includes simple statistics about a grid. It contains the maximum, minimum, mean, and standard deviation of values in the grid.

GEO's GIS SOAPBOX (and list of Helpful Reminders)

1. Map scales should be appropriate and similar.
2. Pay attention to projection.
3. Data can be right (precise) and in the wrong location (not accurate).
4. Data do not improve through GIS processing.
5. GIS output is only as good as your worst data layer.
6. If you're going to say anything about the significance of your GIS analysis, learn and use spatial statistics.
7. Be a resource specialist who uses GIS.

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