Wednesday, September 23, 2015

Data formats in a GIS

The vector formats used in a GIS are listed below:

  1. Postscript is a page definition language that is usually used to export or print a map. It supports graphics in both vector and raster formats.
  2. Digital Exchange Format (DXF) is an external format used for transferring files between computers or between software packages. It is produced by AutoCAD. It does not have topology, but offers good detail on drawings, line widths, styles, colour and text. DXF is typically constructed in 64 layers and each layer consists of different features allowing the user to separate features. The main disadvantage of this format is that it lacks topology and spatial analysis.
  3. Digital Line Graph (DLG). These are distributed by the government and features are in separate files that most GIS packages will import. Extra data manipulation is often necessary. DLGs consist of line work with contour lines removed.
  4. TIGER (Topologically Integrated Geographic Encoding and Referencing system) was designed for use with the 1990 U.S. census. In this system, points, lines and areas can be explicitly addressed. It does not rely on adjacency information or graph theory. Real-world features such as meandering streams and irregular coastlines are given a graphic portrayal that is more representative of their true geographic shape. TIGER files are useful in non-census related research.
  5. Shapefile A shapefile is a vector data format for storing the location, shape and attributes of geographic features. A shapefile is stored in a set of related files and contains one feature class.
  6. Scalable Vector Graphics An SVG is an image that is an extension of the XML language. Any program that recognizes XML can display the SVG image. The scalable part of the term emphasizes that it is possible to zoom-in on an image without loss of resolution. SVG files have the added advantages of being smaller and arriving faster than conventional image files such as pdf, gif and jpeg
  7. Arc-info coverage This is a data model for storing geographic features using ArcInfo software. A coverage stores a set of thematically associated data considered to be a unit. It usually represents a single layer such as soils, streams, roads or land use. In a coverage, features are stored as both primary features (points, arcs, polygons) and secondary features (tics, links, polygons). Feature attributes are described and stored independently in Feature Attribute Tables
  8. ArcInfo Interchange file (e00) An ArcInfo interchange file is also known as export file. It is used to enable a coverage, grid or TIN and an associated INFO table to be transferred between different machines. ArcInfo Interchange files have a .e00 extension which increments to .e01, .e02 and so on if the interchange file is composed of several separate files.
  9. Geo-database A geo-database is an object oriented data model that represents geographic features and attributes as objects and the relationship between objects is hosted inside a relational database management system. A geo-database can store objects, such as feature classes, feature data sets, non-spatial tables and relationship classes.
The raster formats used in a GIS are listed below:
  1. Standard raster format: Many of the formats are based on photographic formats. The file structure has a header with a fixed length and a keyword to identify the format. In the header, the length of one record in bits and the number of rows and columns can be found. Frequently the header has a color table that specifies the colours to be projected.
  2. Tag Image File Format: This format is associated with scanners. It can use run length and other image compression schemes. It is not limited to 256 colours like a GIF.
  3. GEO-TIFF: As a part of header in TIFF format it puts lat/long at the edges of the pixels.
  4. Graphic Interchange Format (GIF): This format for image files is widely used on the internet. It is well suited for images with sharp edges and relatively few gradations of colour.
  5. Joint Photographers Experts Group JPEG is a common picture format. It uses a variable resolution compression system offering both partial and full resolution recovery .
  6. Digital Elevation Models: DEMs have two types of displays - a 30 meter elevation data from 1:24,000 seven-and-a-half minute quadrangle map and 1:250,000 3-arc-second digital terrain data. DEMs are produced by the National Mapping Division of United States Geological Survey.
  7. Band Interleaved by Pixel (BIP) and Band Interleaved by Line (BIL): BIP and BIL systems are produced by remote sensing systems. The primary difference between them is the technique used to store brightness values captured simultaneously in each of several colours or spectral bands.
  8. RS Landsat: Landsat satellite imagery and BIL information are used in RS Landsat. In one format, using BIL, pixel values from each band are pulled out and combined. Programs that use this kind of information include IDRISI, GRASS and MapFactory. It is relatively easy to exchange information from within these raster formats.