This guide introduces basic concepts and functions of Geographic Information Systems (GIS's), and supports functional training in AtlasGIS 2.0 for DOS, a commercial GIS software package. We have chosen AtlasGIS version 2.0 for DOS as an introductory vector GIS package because it combines good functionality, a smooth learning curve and low cost. This guide covers the full range of functions supported by this version. Although most students may never need GIS functions beyond those supported by Atlas, we will also review some advanced GIS functions supported by other GIS packages--time permitting.
This guide has been developed for intensive hands-on instruction in AtlasGIS, both for occasional 5-day GIS short courses offered by the Spatial Analysis Lab, and for FREC 480, "Geographic Information Systems in Natural Resource Management." The FREC 480 course also uses a companion guide to raster GIS principles which supports functional training in IDRISI version 4.1, another DOS-based GIS.
Many of the exercises included in this guide use Delaware data developed by the Spatial Analysis Lab. We also encourage you to use this guide as a quick reference for specific GIS techniques and AtlasGIS functions. Readers are also encouraged to familiarize themselves with the full range of documentation Strategic Mapping, Inc. includes with AtlasGIS, particularly Learning AtlasGIS (spiral-bound) and Using AtlasGIS (notebook).
A geographic information system stores, manipulates and analyzes heterogeneous, complex data elements. These data typically represent heterogeneous geographic features referenced in space and perhaps time. Typical GIS functions include:
AtlasGIS is a pure vector GIS, meaning that it treats geographic features (abstracted to points, lines and regions) as sets of vertices (coordinate pairs) in some geographic coordinate system (lat-long, UTM, state plane, etc.). In contrast, a raster or grid-based GIS treats geographic features as sets of cells in a grid. Vector GIS's are good for modeling discrete spatial phenomena such as roads, power lines, property lines--anything with clearly-defined boundaries. Raster GIS's are good for modeling continuous spatial phenomena such as pollutant concentrations, elevation--anything with diffuse or ill-defined boundaries.
The vector and raster approaches to GIS derive from entirely different origins. Vector GIS's evolved from computer-aided drafting (CAD) technologies adapted to cartography. Raster GIS's evolved from grid-cell interpretation of aerial photos and satellite imagery.
Geographic features can be classed as points, lines or regions.
Wells, houses, and bird nesting sites are examples of point features. A vector GIS represents a point feature as a single (dimensionless) coordinate pair. A raster GIS might represent a point feature as a single grid cell of some (small) dimension chosen by the researcher.
Roads and streams are examples of line features. A vector GIS represents a line feature as a string of coordinate pairs. A raster GIS represents it as a string of grid cells. Some line features close on themselves (e.g., property boundaries) and are sometimes termed "poly-lines." A closed line feature defines a perimeter only. In AtlasGIS, it has length, but no area.
Wetland areas and parks are examples of region features. A vector GIS represents a region feature as a closed string of coordinate pairs defining the region by its perimeter. A region feature has both length (perimeter) and area. A raster GIS represents regions as clumps of grid cells.
Region features can be complex. A single region feature may include multiple "islands," which are separate pieces of the same feature. Or it may completely surround interior "lakes," which are different features.
Where region features share a boundary segment, the sets of vertices defining the shared boundary in the two features should be identical. When digitizing the two adjacent regions, re-digitizing the common border segment usually generates inconsistencies, i.e., gaps assigned to neither region and/or overlap slivers assigned to both regions. Be aware that AtlasGIS does not support after-the-fact resolution of boundary inconsistencies such as these. Its "grab" tool lets you incorporate vertices from adoining features when digitizing new features, however.
The best digitizing strategy follows an arc-node data structure: each boundary segment (arc) is created as a separate line feature with any number of vertices including two end vertices (nodes). Arcs are digitized as line features. Then each region feature is built from the set of arcs, connecting at their common nodes, which bound it. In building a set of contiguous region features, each interior arc will be included in two region feature boundaries; each arc on the external border of the cluster of regions will be included in only one region.
Maps show the relative sizes and positions of geographic features. Curved sections of the Earth's surface are generally transformed through a projection system to representations on a flat map, and each map feature is located in a geo-referenced coordinate system with defined origin and units.
There are many systems for projecting a spherical surface or portion of it onto a flat plane. An ideal projection would accurately represent all the spatial characteristics of the projected features (shape, size, relative positions and distances) using a consistent scale, but no such projection system exists. Actual projection systems are based on various trade-offs between accuracies of position, shape and size.
While most coordinate systems are flat grids, the lat-long coordinate system references a global surface directly, and is not (strictly speaking) a projection. Its origin is the intersection of the prime meridian (0 degrees longitude) passing through Greenwich, England with the equator (0 degrees latitude). The Earth's 360 meridian lines are about 69 miles apart at the equator and converge at the poles. The 180 parallels are equidistant, with about 69 miles separating each degree. A cylindrical projection with parallel meridians such as lat-long will impart a horizontal stretch to map features near the poles; this is why many world maps make Greenland look bigger than Australia.
The universal transverse mercator (or UTM) system divides the earth by meridian into 60 6-degree grid zones. Coordinates are expressed in meters. Each zone's origin is its western point on the equator. Delaware is in UTM Zone 18.
Delaware uses two different state plane coordinate systems, one based on the 1927 North American Datum (NAD), the other based on the 1983 NAD, expressed in feet. Some 1:24,000 USGS quad sheets for Delaware use the 1927 datum, others use the 1983 datum. These coordinate systems are listed as DE and DE83 in AtlasGIS's set of available projections. In Delaware, a point referenced in NAD83 will be about 208 meters north and 1 meter east of the same point referenced in NAD27.
A datum defines a reference surface for a portion of the Earth, or perhaps the whole Earth. Why the different datums? Map projection problems are complicated by the fact that the Earth is not a perfect sphere at all: its poles are somewhat flattened, its equator is somewhat bulged, and it has a slight pear shape. As surveying techniques have improved, geodesists (people who study the shape of the earth) have refined their models of the Earth's surface. Each datum is based on a spheroid ("ellipsoid"), which is an idealized curved surface on which x,y,z surface reference coordinates and a center-of-the-earth reference point are defined. Like datums, some spheroids are local, or defined for specific regions of the Earth; others are global.
NAD 1927 is based on the 1866 Clarke spheroid. NAD 1983 is based on the 1980 Geodetic Reference System spheroid. The WGS datums of 1972 and 1984, widely used by the Department of Defense, are based on the World Geodetic System 1972 and 1984 spheroids, respectively.
Every map has (or should have) a scale, often shown as a ratio. The 1:24,000 scale used in many USGS quad maps means that one inch on the map equals 2,000 feet (24,000 inches) on the surface of the earth.
AtlasGIS supports a large menu of projections and projection/coordinate conversions (File-Geographic-Tools-Project). You can try out different projections on a familiar map area.
AtlasGIS uses three types of files:
Geographic files contain point, line and/or region features. Each record in a geographic file represents a single feature, and includes a unique _ID field, a _LAYER identifier, primary _NAME and secondary _NAME2 fields, and the feature's _AREA (if a region) and _LENGTH (region perimeter or line length). Note that field names in geographic files all begin with an underscore; this distinguishes them from field names in attribute files (see below) which don't.
While line and region features are actually comprised of multiple vertices, the Atlas geographic file structure doesn't reveal the actual vertices directly. (You can manipulate individual vertices through the Edit-Geographic command menu, however.) A geographic features in AtlasGIS may have a maximum of 4,000 vertices.
A geographic file may contain up to 250 layers with different kinds of features. A layer contains only point or line or region features. For example, you might combine a "County" region layer, a "Roads" line layer and a "Schools" point layer in a single geographic file. AtlasGIS's layer settings screen (Display-Layer-Settings) let you turn layers on or off, change colors, line and point styles, define drawing priorities (which layer should be drawn on or under others) etc.
In reality, each AtlasGIS geographic file is comprised of 4 files in the same directory on your disk. These have extensions *.AGF, *.AIF, *.AGX, and *.ANX, and use Strategic Mapping's proprietary data format. You must be careful to keep all of these files together. Use a wildcard .* extension to copy or erase geographic files: e.g., COPY C:\DATA\MYFILE.* A:
Geographic files may include additional files with extensions *.LYR (layer settings), *.FRE (freehand annotations), *.CPT (digitizer control points), *.CLR (color settings), *.PAG (page layout settings), *.SEL (feature selections) and *.VEW (view settings).
Attribute files store information about geographic features, e.g. the population and racial composition of counties, the address ranges of road segments, the sales volumes of store locations, etc. Attribute records may contain up to 255 fields of additional attributes of geographic features, and are linked to geographic records via a common field (the _NAME or _NAME2 field in the geographic file is typically matched to an ID field in the attribute file). Attribute files are dBase-compatible, and can be edited by almost any database manager program. AtlasGIS includes some basic database management functions, but you will probably want to use a dedicated database manager program to perform more complex data manipulations.
Atlas attribute files may contain 4 types of fields:
Attribute files can be used in thematic mapping. For example, a population attribute can be used to determine the size of the dots representing cities. Or you can divide population by _AREA and shade each region in your map according to its population density.
Attribute file extensions are *.DBF (dBase format data file) and *.FED (attribute field settings).
Datapoint files are a sort of hybrid of geographic and attribute files. They usually contain LON and LAT fields georeferencing them to a a single vertex, plus multiple attribute fields. Datapoint files are also dBase-compatible. They are most useful for "pin-mapping," particularly where data are frequently updated.
Datapoint file extensions are *.DBF (dBase format data file), *.FED (field settings) and *.DGX (datapoint index file).
You can save any combination of a geographic file, an attribute file and/or a datapoint files as a Mapfile. Mapfiles have the suffix *.MAP. Atlas always maintains two .MAP files: CURRENT.MAP and DEFAULT.MAP. You can save any other combination of geographic, attribute and datapoint files as DEFAULT.MAP if you wish, or you can load any other mapfile (say, MYMAP.MAP) when you start AtlasGIS by typing:
C:\AGIS> agis mymap
or, if MYMAP.MAP is in a different directory (say, GISDATA):
C:\AGIS> agis /m c:\gisdata\mymap
If you "save the current map" when quitting AtlasGIS, you can pick up where you left off by loading CURRENT.MAP when you re-start:
C:\AGIS> agis current
When you load AtlasGIS, DEFAULT.MAP loads automatically. This is like typing:
C:\AGIS> agis default
Map files also store map and feature display settings, thematic settings, freehand annotations, etc.
NOTE: AtlasGIS uses files in live edit mode. You should be careful to preserve unedited copies of any important files. When you access files (geographic, attribute or datapoint) to work on, you have 2 options, Use or use_As:
File-[Geographic/Attribute/Datapoint]-Use loads an existing file for direct editing.
File-[Geographic/Attribute/Datapoint]-use_As opens a new copy of an existing file for direct editing; the original is not over-written.
File-[Geographic/Attribute/Datapoint]-Readonly opens an existing file for display or query only, but prevents you from over-writing it.
File-Mapfile-Load opens a set of existing geographic, attribute and/or datapoint files with your previously-defined display and thematic settings. You are opening these files for direct editing.
Structurally, AtlasGIS is built on a database manager foundation. Consequently the linkages between AtlasGIS's geographic and attribute files are superior to the database linkages of most other GIS's. You have direct access to both the embedded attribute fields of the active geographic files and all fields in the active attribute file. The Edit-Attribute-Browse utility displays and lets you edit a table of combined geographic/attribute records for all features. This utility includes various database tools for viewing, searching, sorting, selecting and editing records. The Edit-Attribute-Popup utility displays and lets you edit records for one feature at a time.
Each record for a complex (line or region) geographic feature actually contains multiple vertices. Compared to most other GIS's, AtlasGIS provides only limited access to these data at the sub-feature level. You can display the vertices comprising geographic features in Draft mode (Display-Mode-Draft) if you have configured the program to do so (set Vertex Dots to "Yes" in Configure-Program), and you can edit individual vertices with the mouse or digitizing tablet, but you cannot edit vertices in tabular form.
The AtlasGIS "quick reference" sheet provides an overview of the entire command menu structure. The primary AtlasGIS menu is:
File - View - Select - Edit - Operate - Thematic - Display - Configure - Help
Each menu choice has a unique capital letter. You can access a sub-menu or function by simply typing that letter (lower-case), or you can click on that choice with the mouse (using the mouse tends to be slower). The ESC key backs you out of sub-menus. Some frequently-used sub-menus and functions can be accessed from multiple places in the AtlasGIS menu tree.
This guide specifies AtlasGIS command sequences as strings of italicized commands separated by hyphens. You will quickly become adept at frequently-used command sequences. For example, View-Map-Redraw (typed "vmr") updates your map on the screen. Display-Layer-Settings (typed "dls") accesses the main map display controls.
Many Atlas command sequences call up blue control screens. You can change entries in any light-blue cell of a control screen. The F2 key or space bar will let you edit the current (gray-highlighted) cell. The F7 key will clear the cell's editing window.
Some cells in control screens are bracketed with arrows like <<this>>. Hitting the space bar on one of these cells takes you to a pick-list or subsidiary control screen.
The F10 key (or the <<Done>> cell at the bottom of a control screen) closes the control screen.
Before displaying your first map, review the default screen layout.
The command menu runs along the top of the screen. The first line lists current menu choices (functions or sub-menus); the second line shows the highlighted function description or sub-menu choices.
The status area at the right side of the screen identifies the active geographic, attribute and datapoint files (G: A: D:), the last mapfile loaded (M:), the current map projection/ coordinate system (P:), the number of geographic, attribute and datapoint features which are currently selected for analysis or manipulation using the Select command (Gsel: Asel: Dsel:), the map scale (Scale:) and the current drawing mode--"Final" or "Draft" set with the Display-Mode command (Mod:)
The work area is the white area of the screen. This is what your printer or plotter turns into a hardcopy map. You have total control over the layout of the work area through the Display-Page command menu. The work area has multiple elements which you can display or hide, reformat, move or resize. In Atlas's default work area, the map area is the large area below the title box, and the geographic legend and attribute legend boxes are on the right above the scale box. Feel free to customize this work area to suit your own taste.
Once you have developed a work area layout that you like (perhaps with your name included as a Display-Freehand text item), you can save the layout with File-Mapfile-Save and load it each time you begin a work session.