Global Mapper and Shapefiles

http://geographika.co.uk/wp-content/uploads/globalmapper/blue_marble_europe_small.jpgI recently had a small project that required vector features in several thousand Global Mapper project files to be extracted into shapefiles.

The batch export tool didn’t seem to be running on the project files, so I was asked to take a look at the Global Mapper scripting documentation. The syntax was easy to follow, and scripts can be run by passing the script file name as an argument on the command line to the Global Mapper .exe file.

However when looking at the attributes of some of the exported shapefiles in ArcCatalog, the table was blank. Upon further investigation a couple of interesting issues came to light.

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Automated WMS Testing with Python


This post and associated script details how to automate testing WMS (Web Mapping Service) layers. When you’ve set up several MapServer or GeoServer instances with 100s of layers, its nice to be able to make sure everything continues to run smoothly.

The automation makes maintenance proactive, rather than reacting to client’s emails wondering where their data has gone, and it beats pasting long obfuscated URLS into a browser and repeatedly pressing F5..

This script combines three of my current favourite Python libraries.

  • OWSLib a Python OGC Web Service utility library
  • PIL – The Python Imaging Library (PIL) used for image processing
  • Requests – a Python library to make web requests – “HTTP for Humans”

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Building MapServer & MapScript on Windows

imageThere are already Windows packages available for MapServer ready for deployment. However if you need to make a modification to the source files, apply a patch, or just want to see how MapServer is made then you can build MapServer from its C source files. Thanks to the ready made build kits this process should only take 10-15 minutes from start to finish.

Prerequisites: Microsoft Visual Studio with C++

1. Download the MapServer SDK (Software Development Kit) from http://www.gisinternals.com/sdk/. I’m using Visual Studio 2010, and building on a 32-bit Windows machine (still using XP..), so I’m using release-1600-dev

These build kits, provided by one of MapServer’s developers Tamas Szekeres,  contain everything you need to compile both MapServer and GDAL. Checking out all the files from the MapServer SVN repository is not enough, as this does not contain the GDAL or RegEx dependencies (DLLs that MapServer depends on), or SWIG – a tool used to create MapScript files for each of the different scripting languages available in MapServer such as Python, Ruby, PHP, Perl etc.

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OpenLayers and Versioning in Aptana Studio

imageThis post details how to add a Subversion (SVN) plugin to Aptana to allow you to compare OpenLayers code with previous versions.

A straw poll was taken on the OpenLayers mailing list in April 2010, and OpenLayers v3 development is taking place in git, with the central repository stored on GitHub. This means working with SVN and OpenLayers may become redundant. However as many OSGEO projects are stored in SVN with no current plans to move the same plugin and knowledge of working with SVN will still be useful.

For further details on working with SVN there is a free e-book “Version Control with Subversion” available at http://svnbook.red-bean.com/

Installing the Plugin

1. Open the Plugin Manager, by clicking on the green jigsaw piece in the toolbar below.

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MapServer, OpenLayers and the WFS Maze

imageFirst a little background on what I was trying to achieve. I am developing a GIS that has both a web and desktop component. It’s built using SQL Server 2008, MapServer, OpenLayers, and using MapInfo and QGIS for desktop connections.

On the web system I have an OpenLayers map, with an OpenStreetMap background layer. As with nearly all the online tiling services, these are projected in EPSG:900913 (the Web Mercator / Google projection).

I wanted to display a road network on top of these as a WFS (Web Feature Service). The source data is defined in the Irish National Grid projection (EPSG:29902).

At the same time I also wanted people to be able to connect to the WFS via a desktop GIS client using the Irish National Grid projection. Read more



Using ArcObjects and .NET in Python

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This question was posed on the GIS Stack Exchange, and the replies include a number of interesting approaches. Below is another one.

Why not just use .NET?

I have several years worth of DLLs built using VB.NET and ArcObjects. Recently I’ve been using more and more open-source libraries, the majority of which can be manipulated using Python.

A major advantage of using a interpreted language (rather than one that needs to compile) like Python is that you can easily change paths and variables in a script.

You can deliver a folder of .NET ArcObjects DLLs, a sample Python script and an end user can automate their workflow themselves. This negates the requirement to build a UI on top of your code as a (Python familiar) user can script it themselves.

DLLs still have their place as ArcObjects includes many functions not available in the ESRI Python libraries. It is also easier to write and test ArcObjects code in Visual Studio with all the advantages of error checking and auto-completion. Read more



Accessing Cross Domain Data with YQL

imageThe same origin policy prevents code from one domain accessing data from a different domain. For a mapping site requests for KML, GeoRSS, WFS services, and some WMS operations are all affected by this policy, and therefore require a range of workarounds, usually involving a proxy.

One solution is the ExtJS ScriptTagProxy that can be used to retrieve data from an external domain. However for this to work the server must return executable JavaScript code. For example to access an external WMS capabilities file you’d need to set up a special handler on your server to wrap the data in JavaScript before being added to your web page. This pattern is referred to as JSONP (JSON with padding).

YQL

Thanks to this Unwritten Guide to Yahoo Query Language it became apparent you can get Yahoo to automatically do this wrapping for you. Whilst using YQL is still technically a proxy, it’s a proxy you don’t have to worry about maintaining. Read more