The new labeling setup in QGIS has been around for over a year now, and in the upcoming version it will become the default, replacing the old labeling. This new engine brings some advanced options that are quite worth learning, such as bulding labels from expressions, and conditional labeling. I’ll expand on some of these tricks that have already appeared in other QGIS blogs
Continue reading New Labeling in Quantum GIS
In hydrology, a stream network is composed of segments or “reaches” which are arranged in a hierachy. There are several systems of ordering the stream reaches, the most popular of which is the Strahler or Horton number. GRASS GIS offers, alongside the watershed delineation tool r.watershed (discussed here), a set of addons for stream network analysis. We’ll examine how to use these addons, and how to use strahler ordering to improve the visual effect of a stream network map.
Continue reading Strahler stream order in GRASS
There are several PostGIS functions floating around to calculate the UTM zone EPSG code for points in Latitude/Longitude WGS84. However, Spatialite, based on Sqlite, does not support user created functions. So how can we get the same results in a Spatialite database?
Continue reading From Lat/Lon to UTM zone in Spatialite
I returned from a short bike outing with my ride captured as a GPS track. Along the way, I also grabbed the rest stops as waypoints. Both of these were downloaded from the GPS as *.gpx files. So I have tracks.gpx and waypoints.gpx. Now I want to push these layers straight into Spatialite, and do some calculations.
Continue reading Manipulating GPS tracks in Spatialite
Quantum GIS is marching ahead and the latest stable version, 1.8.0, provides some new features and improvements. RPM packages are already out for Fedora, but not yet for the Red Hat 6 clones like Scientific Linux 6. Compiling on your own isn’t hard, so if you don’t want to wait for the official package, here are a few pointers.
Continue reading Getting QGIS 1.8 running on Scientific Linux 6
Users of Ubuntu who want to use Spatialite for managing GIS layers are in luck. New updated packages have been released bringing to Ubuntu and derivatives all the new feature of spatialite 3.1.0.
Continue reading Spatialite on Ubuntu
Users of SQL databases often get tripped up when trying to update values in one table from another table. The method involves a subquery in the UPDATE statement to extract the values from the other (source) table. We’ll review how it’s done, both with simple attribute values, and with an update based on a spatial query.
Continue reading Spatialite- Updating one table from another
Spatialite, like any good spatial data management system, can build a spatial index for your layers. Using this index in your spatial queries will dramatically shorten the runtime for that query. The latest version of Spatialite offers a nice compact format for using a spatial index. To demonstrate, I created a point layer of 20,000 theoretical store locations, with sales data for each store, and a polygon layer of over 280 “local councils”. My mission it to sum up the total sales in each local council. So I need to find which stores are located in each local council and aggregate sales for those stores.
Continue reading Spatialite: Speedup your query with spatial indexing
Creating a random set of points is a standard GIS technique, used often for setting up sampling or monitoring locations. Among the tools in FOSS GIS software which offer this function are QGIS (in Vector -> Reseach Tools -> Random points) or GRASS (using the
v.random module). But suppose you need points spread randomly along a line feature? The R-project package of spatial functions called ‘sp‘ can do just that.
Continue reading Sampling points along a line with R
I visited the growing acacia tree seedling, and took some measurements and photos this week. Here are the results compared to last year.
Continue reading Acacia seedling update