Hillshade modelling is a standard form of terrain representation in cartography. The idea is to simulate lighting of a terrain from a certain direction (or multiple directions). The method is well known and constantly improved in GIS – as means of cartographic representation. It seems, indeed, difficult to imagine spatial analysis on shadows. However, hillshading … Continue reading Some thoughts on hillshade models for Lidar analysis
Depth below horizon: new (old) functionality for QGIS viewshed analysis
The basic idea of visibility analysis is to test whether particular locations are theoretically observable from a given observer point (« theoretically » because our data and algorithms can never ideally replicate real-world situations). Such a query yields a yes/no answer for each tested point, which is typically represented as Boolean raster in GIS software. … Continue reading Depth below horizon: new (old) functionality for QGIS viewshed analysis
Installing Python packages in QGIS 3 (for Windows)
Python is in the heart of QGIS (or in the guts if you prefer), which enables us to use tons of third party Python libraries. In Linux systems, QGIS will use the main Python installation, but in Windows things get more complicated. QGIS has it's own Python, which means we end up with various Pythons … Continue reading Installing Python packages in QGIS 3 (for Windows)
Visibility analysis for QGIS 3.0 : tackling big data
Viewshed plugin for QGIS is based on Numpy library for handling raster data. Numpy is tuned for high performance and is (relatively) easy to use. However, it relies heavily on available live memory and will break with voluminous datasets. Which is what elevation models usually are... Version 0.6.4 introduces an addition to the original algorithm … Continue reading Visibility analysis for QGIS 3.0 : tackling big data
Land visibility in the Mediterranean: a large scale model
Visibility of land was a major consideration for ancient seafaring, for reasons of navigation and security. Even if the open sea travel must have been practised already by the Neolithic period, judging by the colonisation of remote islands and by the exchange of artefacts across the Mediterranean, the sight of the land remained a much … Continue reading Land visibility in the Mediterranean: a large scale model
Natural places and archaeology
First time I heard of something like “natural places” was some ten years ago at a conference in Slovenia. Several, mostly young researchers were proud to demonstrate an existence of non-man-made features on their sites, features that apparently gained a symbolic value before or during human habitation. For instance, trees which were incorporated in Neolithic … Continue reading Natural places and archaeology
Lissage du modèle numérique de terrain BD ALTI
Le problème n’est pas nouveau, mais on ne trouve toujours pas de solutions simples pour le « maquillage » du modèle numérique de terrain (MNT) très couramment utilisé en France, le BD ALTI de résolution de 25 mètres (lien dessus). Celui-ci est disponible en valeurs entières, ce que lui donne un aspect en marches d’escalier … Continue reading Lissage du modèle numérique de terrain BD ALTI
Benchmarking viewshed algorithm: Nuraghi of Sardinia
The new viewshed plugin for QGIS 3 features an improved algorithm which should result in perceptible improvements in terms of efficiency. So let's give it a try! Nuraghe in Toralba-Sant'Antine © Michel Royon / Wikimedia Commons The test data I'm using are the famous nuraghi of Sardinia, drystone towers dating to the 2. millennium BC. … Continue reading Benchmarking viewshed algorithm: Nuraghi of Sardinia
Le géosymbole vu par l’archéologie
Le concept du géosymbole est issu du mouvement culturaliste en géographie, suite au désenchantement, dans les années 80, par les approches structuralistes. Ces dernières donnaient l’impression de relégation de l’humain en marges du grand tableau socio-politico-économique : le discours géographique risquait de prolonger celui du pouvoir qui écrase l’individu. Ainsi, la géographie s’est à nouveau … Continue reading Le géosymbole vu par l’archéologie
Looping through numpy arrays (e.g. moving/rolling window)
Numpy is the cornerstone of matrix based calculations in QGIS (and elsewhere). It does wonders with raster data (unless it hits the limit of available live memory...). A recurrent problem with Numpy is the implementation of various looping routines, such as the sliding window which is frequently used in image filtering and other approaches focused … Continue reading Looping through numpy arrays (e.g. moving/rolling window)