GIS, or geographic information systems, are computerized tools used to store, visualize, analyze, and interpret geographic data. Geographic data (also called spatial, or geospatial data) identifies the geographic location of features making these features "spatially-aware."
Spatial-awareness is not a requirement for solving problems in hydrology but since hydrology is inherently geographic, spatial-awareness can help solve some hydrologic problems more efficiently.
Spatial Reference Frameworks
All geospatial hydrologic algorithms fundamentally rely on the calculation of distance and area. Therefore, geographic (unprojected) coordinate systems (e.g., Latitude/Longitude) cannot be used, as the distance represented by a degree changes depending on the location of the Early's surface. For distributed modeling, area-preserving projections such as the Albers Equal Area Conic are highly recommended to ensure that mass balance (precipitation volume = area x depth) is conserved evenly across all grid cells.
The Coordinate System can be used to set, view, or reproject the coordinate system for a Basin Model. Available options to set the coordinate system include:
- Select a predefined coordinate system: Standard Hydrologic Grid (SHG) or Universal Transverse Mercator (UTM).
- Browse to GIS data with a coordinate system specification (e.g. shape file, raster data, .prj file, etc.).
- Enter the coordinate system specification in the test field using an OGC WKT format.
Terrain Conditioning and Sink Removal
Raw DEMs derived from LiDAR or radar often contain artificial depressions (sinks or pits) caused by data resolution limits, vegetation anomalies, or infrastructure (e.g., bridges acting as dams). If left untreated, flow routing algorithms can trap water in these sinks, terminating the flow path prematurely.