GIS Data is collected by data layer or "theme". . . | |||||||||||||||||||||
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. . . such as boundaries. | "Themes" define features using one or more feature "types". |
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Roads are defined by LINE features. | ||||||||||||||||||||
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These Air Quality Pollution Discharge Locations are defined by POINT features. |
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Hydrographic (water) features are defined by both LINE features (for streams), and AREA features (for lakes and ponds). |
Each individual feature is uniquely numbered, and this number identifies a row in a table where the feature's attributes are stored. |
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The "themes" are then layered on top of each other ... |
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When viewed in a GIS, these layers visually appear as one graphic, but are actually still independent of each other. This allows changes to specific themes, without affecting the others. |
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Overlaying themes creates "maps" |
What are Spatial Data? | Back to Page 4 |