Cape Town user group meeting - November 2018

SA QGIS User Group meeting – 23rd November 2018

The meeting, hosted by NGI at their Mowbray offices and attended by eight people, started late due to delays caused by Cape Town's notoriously bad traffic. After tea and biscuits Immo blecher, the meeting organiser, welcomed everyone and introduced the first of the day's speakers.

Technical Talk 1 - Kevin Musungu: Change Detection in the Philippi Horticultural Region of Cape Town using LANDSAT Imagery

Kevin presented results from one of his recent research projects. In this study, Landsat 5 images from 1990 and Landsat 8 images from 2016 were classified using the QGIS Semi-Automatic Classification (SCP) plugin. Four main classes were identified in the classification, namely urban, bare ground, water and vegetation. Classification results between the two sets of images were analysed and these interpretations were presented by Kevin.

Technical Talk 2 - Immo Blecher: Using the Atlas feature to present groundwater quality per Western Cape primary catchment with feature count - a challenge

Immo demonstrated a project he's involved in where he used the QGIS Atlas feature to map borehole groundwater quality in the Western Cape. A separate map was created for each catchment. Immo demonstrated some of the techniques he used to created the maps, particulary the code used to display the borehole feature counts in the catchment under consideration. This was a work-around he had to develop as the QGIS Show Feature Count function doesn't appear to work when features are filtered for display in the Atlas.

Technical Talk 3 - Dean Ollis: A few tips and tricks in accessing and working with freely available biodiversity data in QGIS

Dean Ollis, a fresh water ecologist, gave a demo of some very useful, freely available environmental datasets:

He followed this by demonstrating how to link QGIS directly to some of these online data sources by means of WMS connections. Dean then demonstrated a QGIS plugin called Species Explorer that can be used to download species data from the GBIF database.

The final part of Dean's talk looked at a function that is available in ArcMap but which doesn't appear to be available in QGIS, namely overlaying polygons on a raster and performing a cross-tabulation of pixel values versus polygons. The QGIS Zonal Statistics function comes closest but provides statistics per polygon rather then per pixel class in each polygon.


A light lunch consisting of pizza was taken.

Technical Talk 4 - Frank Sokolic: Georeferencing and Mosaicking Aerial Photographs

Frank demonstrated the features available in the QGIS Georeferencer and how these can be used to georeference aerial photographs. The process of selecting photographs from a flight plan was discussed as was the difficulty of identifying ground control points (GCP) on old photographs. There was also some discussion on how many GCPs to use and which Transformation formula to apply. The talk ended with a look at methods of balancing the brightness and constrast of the georeferenced photos and how to reduce the visible boundary between adjacent photos.

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The South African QGIS User Group is an open community of professionals and enthusiast who use QGIS..


Immo Blecher.
email: immob19 [AT]

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