Contours are lines connecting points of equal value that represent continuous phenomena (Huisman & A. de By, 2009). Contours are called isolines, but may also have specific terms depending on the phenomenon. These are called contour lines (Figure 45) when used to represent elevation, isobars for atmospheric pressure, isohyet for precipitation, isotherms for temperature and isochrons for time (Olaya, 2014).
The process for generating isolines of all kinds is similar. If you need to create contours, you need a DEM as input information. The isolines are generated with the Contour tool, located at:
ArcToolbox > Spatial Analyst Tools > Surface
The settings of the Contour tool parameters (Figure 46) are detailed below:
- Input raster: Select the raster containing the values (to generate contour lines a DEM raster is required, isohyets a precipitation raster, for isotherms a temperature raster).
- Output polyline features: Select the directory or geodatabase to store the line layer (level curves).
- Contour interval: Define the interval between isolines. It is important to take into account the resolution of the raster; for example, if the DEM has a resolution of 30 meters, it is not recommended to create contours of less than 30 meters.
- Base contour: Optionally, a base level value can be assigned.
- Z factor: Optionally, a conversion factor can be assigned to generate the isolines. This is used to perform unit conversions (e.g. meters to feet).
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