Category Archives: gis

March 20, 2013 · 11:04 pm

Convex Hull, one algorithm implementation

chull
I talked about convex hulls some time ago in an alpha shape post.

The convex hull is probably one of the most basic computational geometry algorithms, and because of that it is present in almost, if not all, geometry/cad/gis libraries and software packages. In this post you will find an explanation of one of the existing algorithms to compute it, an implementation with C++, plus a set of scripts to generate various point clouds and the corresponding hulls.

Objective

Describe one of the possible convex hull algorithms
Implement the algorithm in C++
Provide various scripts to generate random point clouds and compute its convex hulls

I won’t lie to you, this post might be boooring ;-).

continue to this long post

3 Comments

Filed under code, gis

Tagged as C, computational geometry algorithms, geometry, gis, incremental algorithm, make, math, python, vectors

July 31, 2012 · 4:32 pm

DBPedia World

Following my DBpedia Europe Countries post… DBpedia World! Bigger and lovely!.

Maps

Click for full zoomed version!

DBpedia 3.7 World points

DBpedia Live World Points as of july 2012

As always. Everything explained in the post

Qgis – Color to layer attribute

Recently I was working with some data layers in QuantumGIS, playing to decide the proper color combination for my visualization. The result had to be presented in a web environment using OpenLayers.

The point is. I define and check the visualization in Qgis, and then I want to export the results to be served as a OpenLayers Vectorlayer, or as a Mapserver layer. Of course, with the color defined in Qgis.

Openlayers and Mapserver give the option to read the color from an attribute value (here and here). Maybe there is an easier way, but in this case I developed a small plugin to export the current visualized color into a specific layer attribute.

Screenshots

: Color to Attribute – Icon in Qgis main screen

: Main window of the plugin. Select layer and attribute

: Creating a new attribute to fill with the color value

: Feature list after adding the new attribute

humm… more

3 Comments

Filed under code, curious, gis, Maps, tools

Tagged as attributes, color, plugin, python, qgis, qgisplugin, qt, quantumgis

June 19, 2012 · 3:18 pm

Europe Countries – DbPedia+SPARQL+Python

Introduction

DBpedia is a project that aims to extract structured information from Wikipedia. They generate RDF information that can be queried in different ways, one of this is using the SPARQL query language.

In this post I’ll show how to query Dbpedia using a python SPARQL wrapper to obtain all the Wikipedia points of places in Europe. The post explains it step by step, you can fork the code at my github.

..
NOTE from the future:
the original code was Europe.py. The closest actual code would be alldbpediapoints.py
..

Results

The result will look something like this:

DBpedia Europe Points

I have time, show me how it’s done

7 Comments

Filed under code, gis, Maps

Tagged as dbpedia, point cloud, python, qgis, script, semantic web, sparql

May 7, 2012 · 6:41 pm

Basic Network Analysis With GRASS

I performed a little network analysis with ArcGIS on the Schools in Enschede. Given a shapefile with roads and the school points I obtained the service areas at 1km and 2km from the schools. Can I obtain the same with GRASS?

Brilliant! I want to read more

18 Comments

Filed under gis

Tagged as analysis, gis, grass, network

April 23, 2012 · 4:57 pm