GeoGeek

How to generate map contour lines (isohyet, isothermal and isobaric)

The contour line on a map represent lines, straight or curved, describing the intersection of a surface by interpolating one or more horizontal planes, forming gradients on the estimated variable or parameter, in order to interpret graphically. For example, a isohyet, isothermal and isobaric representing precipitation, temperature and pressure respectively in a time unit on a surface.

In this post shows the procedure for isolines (isohyets, isotherms, isobars) in ArcGIS, based on meteorological stations distributed around the province of Loja.

To create a isolínea is necessary to have a layer of weather stations to points (recommended minimum three), geo-referenced, and with a defined projection.

Estaciones meteorológicas ArcGIS

If the weather information is within an Excel spreadsheet, you can add in ArcMap with the Add XY Data tool, then export the table as shapefile (Right click on the layer > Data > Export Data).

For interpolating the Spatial Analyst extension is required, go to the toolbox:

ArcToolbox > Spatial Analyst Tools > Interpolation

Interpolation Spatial Analyst Tools
In this section there is a set of tools for interpolation, according to the ArcGIS Resource Center conceptualized as follows:

  • IDW, uses the measured values surrounding the prediction location to predict a value for any unsampled location, based on the assumption that things that are close to one another are more alike than those that are farther apart.
  • Kriging, is an advanced geostatistical procedure that generates an estimated surface from a scattered set of points with z-values. More so than other interpolation methods, a thorough investigation of the spatial behavior of the phenomenon represented by the z-values should be done before you select the best estimation method for generating the output surface.
  • Natural Neighbor,  interpolation finds the closest subset of input samples to a query point and applies weights to them based on proportionate areas to interpolate a value (Sibson, 1981). It is also known as Sibson or “area-stealing” interpolation.
  • Spline, uses an interpolation method that estimates values using a mathematical function that minimizes overall surface curvature, resulting in a smooth surface that passes exactly through the input points.
  Working with Elevation data in QGIS

 

Defined interpolation method (Kriging for this example) open the tool.

Kriging in ArcGIS

In Input point features select the point layer (weather stations). Z value field select field that contains the data of the isolines (precipitation, temperature, pressure, etc.). Output surface raster selecting the directory, you can also make additional adjustments to raster.

Finally it proceeds to create isolines with the following tool

ArcToolbox > Spatial Analyst Tools > Surface > Contour

In the dialog box fill data Input raster select the previously created raster (Kriging), Output polilyne features define the directory to save the shapefile and Contour Interval allows you to set an interval of isolines, this depends on the surface, variance and parameters under technical criteria.

If you require vector polygons is necessary to reclassify the raster (Spatial Analyst Tools > Reclass > Reclassify) and then transform as vector (Conversion Tools > From Raster > Raster to Polygon). The interesting thing is that ArcGIS has a powerful toolset that allow us to reach a goal in various ways.

Map contour lines

Download data example, please help me with writing (comment), my English is not good.

3 thoughts on “How to generate map contour lines (isohyet, isothermal and isobaric)”

  1. BAsavaraj says:

    Thank You !!

  2. Karunakaran Akhil Dev says:

    The description was very useful. Regards..

  3. husam says:

    That’s good

Comments are closed.

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