 | INTRODUCTION TO GIS |
Geographic Information System (GIS) is a set of coded geographical analytical tools to store, analyse and visualize any kind of spatial data referenced to specific coordinate system. There are many kinds of the software packages produced by various companies which are generally referred to as "GIS software". They include graphical, statistical and database management tools. The complexity and power of these tools defines how comprehensive GIS software is.
Prior to computer age, geographic information systems existed as geographic analytical knowledge preserved in manually produced maps with different layers of information. All functions of the average contemporary GIS existed and were in use for a long time, before computers replaced rules and compasses. Manual interpolation of data, conversion of units and coordinate systems required enormous human resources. Final maps were simultaneously "maps" and "data". Map legend had explanation about methods used and data origin. Therefore, geographical data base consisted of this manually combined product.
During computer age GIS development resulted in separation of traditional maps and data. In GIS data are not maps. In GIS data do not have to be "visible" until it becomes necessary to user. Spatial data analysis can be done without visual display of data as soon as data are verified and properly prepared for the analysis. Being separated from the data files, maps can be modified easily in electronic environment by the software functions. This aspect of the software handling requires caution in application and use of the final hardcopy maps.
One of the main GIS components is coordinate system.
Main technical functions of GIS include:
- overlay of spatial data, creation of statistics and summary of spatial variables;
- integration of data with different scales and coordinate systems;
- creation of new information via data integration;
GIS software consists of the three main modules:
data analysis tools;
data management tools, including input and output;
data visualization tools.
Depending on the software type these modules can be accessed by the user via:
commands (typing at the prompt);
separate menues, buttons and "clickable" icons;
separate modules, having combination of menues, buttons and "clickable" icons;
Various software packages (shareware and commercial) are available for review via Internet or specialized journals and GIS publications. GIS modules can be also found within remote sensing software packages.
For the class the main GIS software package is Arcview 3, developed by ESRI, Inc. Supplemental package for the class will be ARCINFO, developed by ESRI, Inc., as well. ARCINFO has command driven modules (and also some customized menu-driven tools) and can be used as a programming language to develop customized GIS application.
Arcview 3 is a modular software where all manipulations are controlled by menues, buttons and "clickable" icons or tools. It has also programming language Avenue that allows user to modify Arcview interface and create customized applications. Both software packages use and create so-called "coverage" and "shapefile" GIS formats. At the same time they can import and export formats used by other software types (see relevant documentation and description on ESRI Web site).
ARCINFO software will be used in the class to demonstrate principles of digitizing and topology creation. Arcview will be used for majority of class projects.
One of the remarkable features of Arcview is its ability to include in software external modules, so-called "extensions". These modules exist in two types - as shareware (i.e. written by GIS "gurus") and commercial (i.e. licensed) software.
Free extensions are available via ESRI site and licensed extensions have to be bought. One of these extensions will be used in class. It is called "Spatial Analyst". The extension will be used in class for creation of digital elevation models, terrain analysis, hydrologicl and hydrogeological modeling. This extension contains interpolation functions and tools to manipulate grids - raster GIS data.
What is "GIS format" ?
GIS format is any digital georeferenced file that would allow user to apply various geographical analytical tool, such as interpolation, distance measurements, data visualization, etc. Usually, three minimum necessary components of the GIS file are:
coordinate pair;
identification number (index);
attribute;
This general definition of GIS format preserved in various GIS software - ARCINFO, ARCVIEW, MAPINFO, GRASS, IDRISI, etc.
Various GIS data formats are usually reffered as "coverage", "shapefile", "Digital Exchange File (DXF)", etc.
Do not mix them! with digital ready-for-printing files containing maps - Encapsulated Postscript File (EPS), RTF, JPEG, TIFF, GIF, etc. These files are print files or image files.
Raster Data Model (GRID)
Main elements:
rows, columns;
geographic extent;
cell size;
cell value;
Advantages:
fast;
simple;
mathematic modeling is easy;
location-specific manipulation is easy;
fast data acquisition (i.e. via satellites, scanners, etc.);
Disadvantages:
large data sets;
changes in the data size leads to the change in data resolution;
coordinate transformation is difficult and time consuming;
raster data are not elegant for the map production;
combination of the raster data modeling results and relevant vector data can disagree spatially;
Vector Data Model. Arc/Node Data Structure.
Main elements:
Topologic Elements: points, lines, polygons;
Structural Elements: nodes, arcs;
Advantage:
more accurate;
little repetition;
efficient in data storage;
very suitable for plotting devices;
topolygy described explicitly;
excellent for map production;
coordinate transformation and rubber sheeting is easy;
Disadvantage:
complex data structure;
do not forgive topological errors;
display and plotting can be time consuming;
not good at representing continuous fields and surfaces;
spatial analysis and mathematical modeling is difficult;
Tables associated with Vector Data Model:
Polygon Topology (Attribute) Table;
Node Topology (Attribute) Table;
Arc Topology (Attribute) Table;
The Triangulated Irregular Network (TIN)
TIN structure is a vector-based topological data model that is used to represent terrain data.
TIN represents the terrain surface as a set of interconnected triangular facets.
TIN structure is a vector-based alternative to the traditional raster representation of terrain surface - Digital Elevation Model (DEM).
TIN is a result of interpolation between measured elevation values. Unlike DEM, composed of regular grid, TIN facets have different sizes depending on data density. Therefore, TIN can describe terrain surface better. At the same time TINs are much more complex than DEMs and their production can be time consuming.
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