GIS in EA 2002-03
J. M. Stroh
Introduction.
For the purpose of this class a geographic information system (GIS) means an automated, computer-based, means of combining spatial coordinates and another type of information.
Think of it as maps and information. We will use GIS for analysis, not just plotting spatial data.
In this class we will use primarily a plane rectangular coordinate system where any spatial location has an X (abscissa) value and Y (ordinate) value making a coordinate system. The coordinate system has on origin at 0,0.
GIS data structures come in two fundamental types, raster and vector.
Raster systems rely on data presented in rows and columns, an array. Each datum (or cell or pixel) in the array has a number assigned to it. This number conveys the information, for example a code referring to lakes or fields, or a number representing elevation or slope. The structure is very simple. Raster systems code points as single cell locations, and lines as "strings" of cells all with the same code.
Vector systems.
Rely on spatial coordinates of points to represent points and coordinates of connected points to represent lines. Areas (polygons) are represented by connected points too, with information about what makes an area. Information called topology (not topography) is essential for a vector system to convey information about lines and areas. Topology includes beginning, direction, left and right, and ending information. All real GIS vector systems include information on spatial coordinates and attributes of features in tables.
A vector system would represent a lake by a closely spaced series of connect points to generate the outline of the lake, and topology would indicate it is a closed area. A road would have connected points but no enclosed area. Each feature, lake, and road would have an attribute table with data as available (for example depth, fish species, microorganisms in the lake).
Generally vector systems work best for line or point data, and situations where analysis of tabular data plays an important role. Raster systems work better with continuously variable data, data already in raster format (almost all remotely imaged data from scanners), and where spatial analysis is most important. At Evergreen we have both capabilities, and our GIS resources support both in a single package, but with one dominating. Idrisi works best with raster data but supports vector data with its database workshop. ArcView works best with vector data but has an extension called Spatial
Analyst for work with raster information.
In environmental analysis what works best? Both. You will find ample applications using both vector and raster based GIS in what is often referred to as distributed hydrological analysis. Distributed just means the data gets split into smaller spatial (landscape) units
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for more precise analysis, or so the theory goes. Generally the partially empirical models like the National Resources Conservation Service's (formerly Soil Conservation Service) rational method or the EPA's Hydrologic Simulation Program FORTRAN (HSPF) use vector polygons and lines readily. The most sophisticated physically based simulation models like those of the TOPOG family, where the GIS does most of the work generating the needed data, use raster systems. You should know both.
Data Entry.
In environmental analysis you will usually have some data available in GIS format, but almost always you will need to enter data yourself. So, we will start GIS in this class with data entry concepts and terms.
Much useful data comes in tables or on paper maps. To convert this to GIS format you do not need to understand how your GIS stores data but it helps. Most of the time you will enter data as tabular information or as vector type map data. Remember these terms and concepts
Spatial data comes associated with points, lines, or areas. In GIS jargon these are called points, lines or arcs (arcs have topology) and polygons.
We define a node as a point vector data structure that begins or ends a line (arc); or acts as the intersection of two or more lines.
We define a vertex as a point that help define the shape of a line (many = vertices).
Topology means the explicit rules defining the linkages of the spatial geographic elements. A computer needs these explicit rule for map relationships that we take for granted.
Remember these terms : raster, vector, topology, point, line, area; point, arc, polygon; nodes, vertices. A point has one node; an arc has two nodes; a line has at least two nodes; a polygon has one special node (begins and ends at the same place.
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