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Geographical Data Sets
The size of the cells in a raster file is an important factor. Smaller cells improve image quality because they increase detail. As cell size increases, image definition decreases or blurs. In the example, the position of the line's edge is defined most clearly if the cells are very small. However, there is a trade-off: Dividing the cell size in half increases file size by a factor of four.
Cell size in a raster file is referred to as resolution. For a given resolution value, the raster cost does not increase with image complexity. That is, any scanner can quickly make a raster file. It takes no more effort to scan a map of a dense urban area than to scan a sparse rural one. On the other hand, a vector file requires careful measuring and recording of each point, so an urban map will be much more time-consuming to draw than a rural map. The process of making vector maps is not easily automated, and cost increases with map complexity.
Because raster data is often more repetitive and predictable, it can be compressed more easily than vector data. Many raster formats, such as TIFF, have compression options that drastically reduce image sizes, depending upon image complexity and variability.
Raster files are most often used:
- For digital representations of aerial
photographs, satellite images, scanned paper maps, and other
applications with very detailed images.
- When costs need to be kept down.
- When the map does not require analysis of
individual map features.
- When "backdrop" maps are required.
In contrast, vector maps are appropriate for:
- Highly precise applications.
- When file sizes are important.
- When individual map features require
analysis.
- When descriptive information must be stored.
Raster and vector maps can also be combined visually. For example, a vector street map could be overlaid on a raster aerial photograph. The vector map would provide discrete information about individual street segments, the raster image, a backdrop of the surrounding environment.
Digital Map Formats- How Data Is Stored
The term file format refers to the logical structure used to store information in a GIS file. File formats are important in part because not every GIS software package supports all formats. If you want to use a data set, but it isn't available in a format that your GIS supports, you will have to find a way to transform it, find another data set, or find another GIS.
Almost every GIS has its own internal file format. These formats are designed for optimal use inside the software and are often proprietary. They are not designed for use outside their native systems. Most systems also support transfer file formats. Transfer formats are designed to bring data in and out of the GIS software, so they are usually standardized and well documented.
If your data needs are simple, your main concern will be with the internal format that your GIS software supports. If you have complex data needs, you will want to learn about a wider range of transfer formats, especially if you want to mix data from different sources. Transfer formats will be required to import some data sets into your software.
Vector Formats
Many GIS applications are based on vector technology, so vector formats are the most common. They are also the most complex because there are many ways to store coordinates, attributes, attribute linkages, database structures, and display information. Some of the most common formats are briefly described below
Common Vector File Formats
| Format Name |
Software Platform |
Internal or Transfer |
Developer |
Comments |
| Arc Export |
ARC/INFO* |
Transfer |
Environmental Systems Research Institute, Inc. (ESRI) |
Transfers data across ARC/INFO* platforms. |
| ARC/INFO* Coverages |
ARC/INFO* |
Internal |
ESRI |
|
| AutoCAD Drawing Files (DWG) |
AutoCAD* |
Internal |
Autodesk |
|
| Autodesk Data Interchange File (DXF™) |
Many |
Transfer |
Autodesk |
Widely used graphics transfer standard. |
| Digital Line graphs (DLG) |
Many |
Transfer |
United States Geological Survey (USGS) |
Used to publish USGS digital maps. |
| Hewlett-Packard Graphic Language (HPGL) |
Many |
Internal |
Hewlett-Packard |
Used to control HP plotters. |
| MapInfo Data Transfer Files (MIF/MID) |
MapInfo* |
Transfer |
MapInfo Corp. |
|
| MapInfo Map Files |
MapInfo* |
Internal |
MapInfo Corp. |
|
| MicroStation Design Files (DGN) |
MicroStation* |
Internal |
Bentley Systems, Inc. |
|
| Spatial Data Transfer System (SDTS) |
Many (in the future) |
Transfer |
US Government |
New US standard for vector and raster geographic data. |
| Topologically Integrated Geographic Encoding and Referencing (TIGER) |
Many |
Transfer |
US Census Bureau |
Used to publish US Census Bureau maps. |
| Vector Product Format (VPF) |
Military mapping systems |
Both |
US Defense Mapping Agency |
Used to publish Digital Chart of the World. |
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