M@IN - Accessing Spatial Data Tutorial
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Data represent the second key component of GIS technology. Spatial data may be thought of as features located on or referenced to the Earth's surface, such as roads, streams, political boundaries, schools, land use classifications, property ownership parcels, drinking water intakes, pollution discharge sites - in short, anything that can be mapped.
Geographic features are stored as a series of coordinate values. Each point along a road or other feature is defined by positional coordinate value, such as longitude and latitude. The digital spatial data are specially formatted for use by GIS software and cannot be read or used by other software.
The GIS stores and manages the data not as a map but as a series of layers or, as they are sometimes called, themes. The examples of geographic data cited above are the layers or themes. Take a look at A visual description of GIS data (it will lead back to here).
Within a GIS, attributes are stored with the geographic data. Attributes are simply the information that we know about the geographic data. For example, for a roads data layer, the attributes may include the road name, highway number, speed limit, paving material, maintenance history, or any other information related to the management or use of this particular layer. The GIS automatically calculates measurement information, such as the length of a road segment or the area of a land parcel.
How Do I Find Spatial Data?
Ever since GIS emerged on the scene as a valuable decision support tool, GIS users have struggled with the issue of acquiring good data. The cost of converting map features to a digital format for use in GIS is extremely expensive. The NSDI represents a concerted effort by the GIS community nationwide to facilitate the development and exchange of digital spatial data and thereby minimize the costly duplication of data development activities by disparate organizations.
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