Lab 5 - Interpolation with QGIS

Lab Overview

Objective:

In Lab 5 we explore QGIS, the COMPLETELY FREE & OPEN SOURCE sister-program to ArcGIS. Though there are some differences between the two programs, transition to QGIS was relatively seamless, at least for the simple exercise presented below. Here, using QGIS we interpolate data measurements from a DGPS instrument taken over Lake Arlington, Texas, to create a depth map of the lake.

1.0 Differences between ArcGIS and QGIS

Though both are GIS software and have many similarities, there are important differences in the user interface that should be noted. This site > https://gisgeography.com/qgis-arcgis-differences/ < describes 27 differences, and they're super exited about it! Below, I will outline two of the differences that were most obvious to me during this exercise.

1.1 Adding Data to QGIS

In QGIS, the type of data added must be specified, whereas in ArcGIS, the user can add a layer of any kind without specifying if the layer is a raster layer, a vector layer, a point layer, etc. The interface in QGIS is shown in Figure 1.

1.2 Map Layout View

To access the Map Layout View in QGIS, I followed the path Project > New Print Layout > Import Map. Individual items were added using the Add Item drop down menu. Adding the North Arrow differed most from the same task in ArcGIS. To add the North Arrow, Add Image had to be selected, and once a picture box had been created, a North Arrow had to be selected and configured from the available images in the QGIS directory. Configuring the arrow to True North was similar to the ArcGIS interface. The result of this process can be seen in Figure 9 at the end of this report.

Figure 1: QGIS Add Layer Interface

2.0 Creating an elevation map through interpolation of field depth measurements

The task today is to use measurements from a depth sounder operated over Lake Arlington, Texas to create an elevation relief map and contours (Ghandi, 2019). The following section will describe the steps taken to that end.

2.1 Adding and visualizing data

Data was downloaded from the Texas Water Development Board (2019), including shapefiles representing the boundary of the lake, point files containing depth information, and ..... The data was added using the Add Layer > Add Vector Layer tool, as described above (see Figure 1). The initial appearance of the map is shown in Figure 2, where the points represent depth measurements. Zooming in on the points, as in Figure 3, shows that they are actually individual measurement locations.

Figure 2: Boundary and measurement data (before interpolation) displayed on QGIS

Figure 3: Zoomed in measurement data (taken with DGPS instrument)

Figure 4: Measurement information (visualized using information tool)

2.2 Interpolation

Interpolation is...

TIN, or Triangulated Irregular Network interpolation is applied to the data, as this is the method commonly applied to elevation data. Inverse Distance Weighting (IDW) interpolation also exists within QGIS, but this technique is not commonly used withe elevation, but is more often applied to mineral concentrations, populations, etc (Ghandi, 2019).

The number of rows and columns are specified (4096 and 2502, respectively). X min and max and Y min and max are set by using the "current extent" option. The "Cellsize" option is not available in QGIS 3.4.2, so this was not specified. The output interpolation .tif file is clipped to the lake boundary using Raster>Extraction>Clip by Mask. Figure 5 shows this clipped interpolation output.

2.3 Visualizing interpolation data

2.3.1 Colormaps

Applying a color scale is a useful way to visualize the results of an interpolation. Figure 6 shows one possible coloration scale that allows us to visualize the different depths in Lake Arlington, with red being shallow, and blue shades indicating deeper water. In this case, Singleband pseudocolor was selected, along with the Spectral colormap. The colors were inverted, since we are working on the production of a depth map, rather than a height map.

Figure 5: Result of TIN interpolation, clipped to the Lake Arlington boundary.

Figure 6: TIN Interpolation depth results with colormap applied

2.3.2 Contours

Contours can be used to add further detail to our map, allowing for more direct visual analysis. Raster > Extraction > Contour is followed. The interval between the contour lines is set at 5 (ft), attribute name is set to ELEV (elevation), and the output is shown in Figure 7. Finally, the elevation of each contour can be labeled, using the Label tool in Properties. In this case, the labels were set to curve, based on the ELEV attribute. Figure 8 shows these results. Figure 9 is a presentation of the final map, created with QGIS Composer.

Figure 7: Depth with Contours, Lake Arlington, Texas

Figure 8: Depth with contours labeled, Lake Arlington, Texas

Figure 9: Depth Map Layout, created with QGIS

To learn in the future...

create automatic layout templates
export interactive maps
explore other QGIS plugins

References:

Ghandi, Ujaval. (2019). Interpolating Point Data. in QGIS Tutorials and Tips. Retrieved from: http://www.qgistutorials.com/en/docs/interpolating_point_data.html. Accessed on: 06 February 2019
Texas Water Development Board (2019). Lake Arlington in Completed Surveys and Data. Retrieved from: http://www.twdb.texas.gov/surfacewater/surveys/completed/list/index.asp. Accessed on: 06 February 2019.

Done for Advanced GIS for Natural Resource Management, in the McGill University Department of Natural Resource Sciences, Professor Jeffrey Cardille

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