By |

Spatial indexing methods help speed up spatial queries. Most GIS software and databases provide a mechanism to compute and use spatial index for your data layers. QGIS as well as PostGIS use a spatial indexing scheme based on R-Tree data structure – which creates a hierarchical tree using bounding boxes of geometries. This is quite efficient and results in big speedup in certain types of spatial queries. Check out Spatial Indexing section of my course Advanced QGIS where I show how to use R-Tree based Spatial index in QGIS.

If you use Python for geoprocesisng, the GeoPandas library also provides an easy to use implementation of R-Tree based spatial index using the .sidex attribute. University of Helsinki’s AutoGIS course has an excellent example of using spatial index with geopandas.

In this post, I want to talk about another spatial indexing system called H3.

Continue reading →

By |

When working with raster data, you may sometimes need to deal with data gaps. These could be the result of sensor malfunction, processing errors or data corruption. Below is an example of data gap (i.e. no data values) in aerial imagery.

Source Image: © Commission for Lands (COLA) ; Revolutionary Government of Zanzibar (RGoZ), Downloaded from OpenAerialMap. (Note: The data gap is simulated using a python script and is not part of the original dataset)
Continue reading →

By |

Google Earth Engine (GEE) is a powerful cloud-based system for analysing massive amounts of remote sensing data. One area where Google Earth Engine shines is the ability to calculate time series of values extracted from a deep stack of imagery. While GEE is great at crunching numbers, it has limited cartographic capabilities. That’s where QGIS comes in. Using the Google Earth Engine Plugin for QGIS and Python, you can combine the computing power of GEE with the cartographic capabilities of QGIS. In this post, I will show how to write PyQGIS code to programmatically fetch time-series data, and render a map template to create an animated maps like below.

Continue reading →

By |

When you want to buffer features that are spread across a large area (such as global layers), there is no suitable projection that can give you accurate results. This is the classic case for needing Geodesic Buffers – where the distances are measured on an ellipsoid or spherical globe. This post explains the basics of geodesic vs. planar buffers well.

Continue reading →