> Objective

• This section explains how to interpolate points in QGIS 

• This section shows how to interpolate points by using the nearest neighbour interpolation method 

• This section shows how to interpolate points by using the kriging interpolation method 

• The students will also learn how to add values to raster cells without values 

> Index / Table of Contents

• Introduction  

• Tutorial(s)  

  • Nearest Neighbour Interpolation  

  • Kriging interpolation  

  • TIN interpolation  

  • Fill no data raster cells

• QGIS installation  

> Summary and Highlights

Introduction  

With spatial interpolation unknown values of points can be estimated by using points with known values. This section addresses the nearest neighbour interpolation method, which assigns point x with unknown value the value of the closest point y, the kriging interpolation method, which uses statistical models to predict the unknown value of point x, and the TIN interpolation, which is based on triangulation of data points. This section also covers how to fill raster cells without data. 

Tutorial(s) 

Nearest Neighbour interpolation  

This method is the simplest approach to interpolation. The value of an attribute receives the value of the nearest neighbouring point.    

•  Interpolate vector data to raster data using the Nearest Neighbour interpolation and Inverse Distance Weighting methods   Hans van der Kwast: “Spatial interpolation in QGIS”  Language(s): English   Time: 15min 51s  Chapters: Nearest Neighbor Interporlation (start-0:14), interpolations (0:14-0:37), interpolation tools (0:37-2:05), inverse distance (2:05-3:57), transparency (6:25-6:40), blending modes (6:40-8:17), interpret the results (8:17-10:13), 3d viewer (10:13-13:09), Nearest Neighbor (13-09-end)   https://www.youtube.com/watch?v=By-K5C7jDQc

• Interpolate temperature point data to raster using the Nearest Neighbour interpolation and Inverse Distance Weighting methods    Hans van der Kwast: “Interpolate Points to Raster (Thiessen & IDW) in QGIS”  Language(s): English   Time: 7min 36s   Chapters: clean up data (start-0:33), convert temperature to Celsius (0:33-1:15), remove table with temperature (1:15-1:55), interpolate to raster (1:55-2:48), inverse distance weight (2:48-end)  https://www.youtube.com/watch?v=oiIVJdZqO8k 

• Control the output extent and spatial resolution with IDW, Nearest Neighbor interpolation and the graphical model from the PCRaster Resource Sharing repository   Hans van der Kwast: “Nearest Neighbor Interpolation (Thiessen) in QGIS with Control of Extent and Resolution”  Language(s): English   Time: 5min 29s   Chapters: introduction (start-1:02), grid (Nearest neighbour) tool (1:02-1:27), additional command line parameters for gdal_grid (1:27-2:53), add models from PCRaster resource sharing repository (2:53-4:14), Thiessen model from PCRaster repository (4:14-end)  https://www.youtube.com/watch?v=r5vhPxeD-mo

Kriging Interpolation 

This widely used interpolation method uses separate statistical models for each dataset to predict the attribute values.   

• Interpolate points to raster with kriging   Hans van der Kwast: “Interpolate points to rasters with kriging in QGIS using the Smart-Map plugin”  Language(s): English   Time: 4min 17s  Chapters: introduction (start-0:33), install Smart-Map plugin (0:33-2:03), interpolation with Smart-Map plugin (2:03-3:43), results (3:43-end)  https://www.youtube.com/watch?v=-Vv1cAhiSVk

TIN interpolation 

The TIN interpolation method is based on the triangulation of data points. The attribute values are determined by the values of the points that form the triangles in which they are placed. A linear interpolation is performed with these points to find the attribute values.    

• Create an elevation relief map and contours from field depth measurements for Lake Arlington in Texas  Ujaval Gandhi: “Interpolating Point Data (QGIS3)”/”Interpoleren van puntgegevens (QGIS3)”  Language(s): English, Dutch  Chapters: overview of the task, get the data, procedure   EN: https://www.qgistutorials.com/en/docs/3/interpolating_point_data.html   NL: https://www.qgistutorials.com/nl/docs/3/interpolating_point_data.html

Fill no data raster cells  

Sometimes there are raster cells without data. The following tutorials explain how to add values to them.  

• Interpolate voids (nodata pixels) in a Digital Elevation Model (DEM)  Hans van der Kwast: “Interpolate voids in DEM for catchment delineation in QGIS”  Language(s): English  Time: 4min 56s   https://www.youtube.com/watch?v=FwJSBoTn1y4  

• Replace NoData raster cells with values from other raster layers  Hans van der Kwast: “Dealing with NoData in Discrete Rasters using QGIS”  Language(s): English  Time: 5min 31s   Chapters: intro (start-0:48), hiding values for visualisation (0:48-1:52), assigning NoData values to rasters (1:52-3:10), calculating the number of data cells (3:10-4:16), replace NoData cells with values from other raster layers (4:16-end)    https://www.youtube.com/watch?v=qApnkq7qsFU  

• Fill no data AHN file  Erik Meerburg (Geogoeroe): “Gaten in een AHN-beeld opvullen (en er daarna nog even mee v”  Language(s): Dutch  Time: 6min 7s   https://www.youtube.com/watch?v=e0bvqQPqi14 

• Fill no data AHN file  Erik Meerburg (Geogoeroe): “Gaten in een AHN-beeld opvullen”  Language(s): Dutch  Time: 6min 15s  https://www.youtube.com/watch?v=86xgzslcoAk

> QGIS Installation

Do you still need to install QGIS? The QGIS installation and update section explains how to install QGIS on Windows and macOS. 

Useful Links 

QGIS Training Manual

QGIS User Guide

GIS OpenCourseWare

Hans van der Kwast YouTube channel

Ujaval Gandhi (QGIS Tutorials and Tips website) 

Erik Meerburg (Geogoeroe) YouTube channel 

#QGIS, #Interpolation, #Nearest Neighbour Interpolation, #Kriging Interpolation, #TIN interpolation, #Nodata values