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Introduction to GIS and formats
GIS
In order to work effectively with large quantities of data the use of geographic information systems (GIS) is necessary.
Geographic Information Systems (GIS) are a computer programs that organize, display, and analyze spatially distributed data. At its simplest, GIS is "mapping", providing a geographic arrangement of extremely diverse information. GIS has been applied in geology, biology, psychology, anthropology etc., and has been around in Crop Forecasting for a long time.
General purpose GIS programs are designed to capture and analyze any kind of geographical data. The most common commercial GIS software packages worldwide are:
Raster formatHowever the open-source community has created a wide range of free tools (to be downloaded from the internet). A nice start page can be found here: http://www.mapcruzin.com/free_gis.htm
Data
GIS data is commonly in one of two forms:
- Raster or image format. A grid of rows and columns of cells. These might represent photographic or scanned images. Within agrometeorology satellite images are usually represented in this way. Each cell represents a value. This value can be assigned a thematic colour in order to visualize the data. Raster data come in a large number of file types. A good overview of raster file types can be found here: http://data.geocomm.com/helpdesk/formats.html#raster-formats
Vector format
- Vector format. Geometric shapes, including points, lines, and polygons. These might represent buildings, roads, and counties. This link gives sums up the most commonly used general purpose vector formats: http://data.geocomm.com/helpdesk/formats.html#vector-formats
Data types used in crop forecasting.
Besides the above-mentioned file types, crop forecasting has its own file types. This has developed over the years, mainly because low-cost GIS systems did not exist at the time most early warning systems were set up. The most important file types are:
- Image data in IDA (Image Display and Analysis) format. The IDA image file consists of a 512 byte image header followed by unblocked (raw binary) image data, 1 byte per pixel, beginning at the top-left of the image and processing row by row. This means that the size of an IDA image in bytes can be expressed as 512 + (lines*pixels). An image that is 150 lines high and 234 pixels wide will have an image size of 35612 bytes. This provides an excellent check whenever images seem to be distorted.
- Vector data in BNA format. This format has been developed by a company (Atlas GIS) for data exchange purposes. It is a simple ASCII format. The format allows the definition of points, lines and polygons, the three basic vector entities. See an example of a BNA file BNAFile.
