Interpolation
with QGIS

 

 

 

 

 

 

Overview

Making an interpolated map with Quantum GIS

Instructions

Import Vector layer (election shapefile) and select directory. Geodatabase and shapefiles are often a folder of files.

Import shapefile from https://www.census.gov/geo/maps-data/data/cbf/cbf_state.html Use medium resolution.

Use this for the State outlines.  Once open in QGIS, set fill as transparent (in Layer Properties/Style/Single Symbol/Fill).

Delete Alaska and Hawaii. To do this, first toggle editing in the layer you want to edit. Then select polygons you want to delete. Save edits; repeat.

Project the map. Select all layers to be projected and set CRS as WGS 84. Then in Project menu/Project Properties select US Contiguous Albers Equal Area projection.

Convert features to point layer using Polygon Centroids (In Vector menu/Geometry tools. The interpolation will be based on points which will be the center of polygons (counties). Always set the input (this would be your election counties data) and output file (your Centroids.shp file).

 

 

Materials

Election data shapefile

Other Resources

QGIS Tutorials

Adobe Illustrator

Interpolate based on the Election layer and Points you created.  The Interpolation tool is found in Raster menu. If not there, go to Manage Plug-ins and add the interpolation tool. Now, in Interpolation dialogue, select Centroids as your input. Choose PerTrump or PerClinton. Add the data. Choose Inverse distance weighting choose an exponent to weight the distances. If you select 2, this means that the distances are squared. Squared distances are the most common for the inverse distance squared method.

Select a cell size of 0.05 x 0.05. If your raster is not covering the entire US states, you must change the extents (x and y bounds).

Next, extract contours. Go to Raster>Extraction>Contour. Select an interval that is appropriate. This will be some fraction of your data range (i.e., max – min). Remember to create output file.

In order to make a proper clipping mask, dissolve your states’ geometries (this gets rid of internal boundaries). Use the Vector/Geoprocessing tools/Dissolve. Next, clip the raster (the interpolation) Go to Raster>Extraction>Clipper

Base the extraction on the Dissolve layer.

Now clip the contour layer. Go to Vector>Geoprocessing>Clip.

Style map so that your contours, interpolation surface, and state outlines are visible.

Then output a pdf to Illustrator for final design layout. Add legend, titles, etc., in Adobe Illustrator. Incorporate some innovative design elements from this page.