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

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)