Watch out for OpenLayer Distances

Apr 292010

imageI had the fortune on my most recent project, a MapFish mapping system, of having someone else do some thorough testing. One thing that had escaped me, was both the measure tool and area tool were returning incorrect results.

The distance as-the-crow flies between Dublin and Cork is 219 kilometres (136.08 miles in old money) wheras my tool was returning 357km. Having never driven this route it didn’t set off any alarm bells. Its also not the sort of function I’d unit test, – one as its in JavaScript, and two as the sample code seemed to work “out-of the box.”

It turns out measurements are planar by default, as briefly referred to in the OpenLayers API docs (although not on the actual measure tools pages..), and there is a geodesic parameter. By default this is set to false, so in a Mercator projected map (so anything using Google, Bing, or OpenStreetMap background mapping) measurement tools will be wrong. Maybe something to check if you’ve created a system – I’ve been 6 months working with OpenLayers and somehow managed to miss this one. Continue reading »

Compiling a 64 bit Version of PyISAPIe

Apr 162010

image Welcome to a veritable novella. I’ll jump straight to the end – there is no happy outcome, as yet, to this post there is a happy outcome! I have compiled a 64 bit PyISAPIe DLL (a program that runs Python at high speed under IIS), and it is up and running on Windows Server 2008 and IIS7. Many thanks to Phillip Stabon, the creator of PyISAPIe, for additional help, and for the latest version of PyISAPIe which makes compilation for different set ups much easier.

Some background information – I’ve not done any C++ for 10 years (and even then my most advanced program was to finish off a code sample on a hotel register for pirates – of the traditional kind). Also I’ve never used Subversion. Hence there are lots of pictures, and every step is detailed. If there’s anything missing let me know.

I’m not sure why I started doing this – the performance of Python using 32-bit version is fine, and since I started working on this Phillip Stabon, has mentioned he will be making a 64 bit DLL himself. However as there are many different versions of Python around, and different GIS tools rely on different versions it seemed a good idea to document how to do this from scratch.

In this example I’m using Visual Studio 2008, and a number of other tools that are detailed as I go along. I’ve also got the 64 bit compilation tools installed. There are a few notes from Phillip on compiling it for Windows 64-bit in the discussion lists (see quoted text underneath the post). Since this post was first published there is a newer discussion in the forums.

Anyway “How to Compile PyISAPIe for a 64-bit Server in 5 Easy Steps!”…

Continue reading »

Logging OpenLayers with Firebug

Apr 132010

imageI take it as given that anyone developing with Firefox and OpenLayers has Firebug installed. Firebug has a number of powerful tools that can make the previous nightmare of JavaScript debugging tolerable.

One feature I recently “discovered” are the logging options. It’s definitely a timesaver over my previous method of putting temporary alert(vals); throughout my code. As well as debugging the logging is very useful for automatically documenting formulae in JavaScript with worked examples that can then be pasted straight into the documentation.

An example of the logging capabilities can be seen on my sample OpenLayers page. If you already have Firebug installed you should be able to see the output similar to that below when the page loads.

image

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Reboot to Refresh Environment Variables

Apr 062010

I wasn’t aware that if you change Windows environment variables that are used by IIS then you need to reboot the machine for the changes to take effect. As described by David Wong

This is because NT services like IIS inherit their environment from services.exe, which does not get updated when you change system environment until you reboot.

This knowledge may save a couple of hours of frustration if you are running Python scripts on IIS through PyISAPIe.

Continue reading »

Setting Up Python on IIS7

Feb 042010

I had already configured Python to run through the Apache webserver on my development server, but after a few issues on the production server (Apache freezing / crashing) I wanted to test running Python scripts with IIS7. The principle aim was to run TileCache through IIS rather than requiring Apache.

Why Not CGI?

While IIS 7 has Fast-CGI installed (see this IIS forum), even better performance can be achieved using ISAPI. This answer from ServerFault has a good summary on why ISAPI should be preferred over than CGI. Not only  performance  should be considered – maintenance should also be taken into account. If a web master or hosting service  is familiar with IIS then they are also likely to be familiar with ISAPI.

From the PyISAPIe site:

The reason ISAPI applications have the capability of being better than CGI or FastCGI applications is mostly due to its tight integration with the web server environment. Instead of initializing an entire program from scratch (in this case, the Python interpreter) every time a request is made for a page, an ISAPI extension only has to provide a function that is called upon every request. For interpreting Python scripts on a per-request basis, this means that the interpreter can be initialized once and used many times, creating a very noticeable performance gain.

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Minifying the Mapfish Client on Windows

Jan 282010

This page has 72 external Javascript scripts. Try combining them into one.

My YSlow Firefox plug-in was telling me that I had included too may JavaScript files, and that they should be combined into a single-file, and minified. The default setup of Mapfish has a single script include (mfbase\mapfish\MapFish.js), but this script  automatically loads around 40 further scripts.

I tried a couple of unsuccessful attempts at copying and pasting various Mapfish JS files into an online JavaScript compressor. However when referencing the minified file I kept getting “undefined” errors from various Mapfish plugins and OpenLayer objects.

Initially I was looking to download a ready-made “compiled” MapFish file, but was unable to find a version online. I have uploaded a zipped version so if you want the file without going through the steps below you can download a copy. Note that if you have made any modifications to the MapFish files then these will not be available, and there are no guarantees it will work.

There are instructions on creating a single-file Mapfish client build on the Mapfish site. This process uses JSBuild which is part of JSTools – “a collection of python scripts for packaging and efficiently serving javascript.”
Unfortunately these instructions require a Linux operating system, and I’m using Windows. I eventually managed to “translate” these into Windows commands, and have documented the steps below.

Continue reading »

Automated WMS Reports

Jan 262010

This post details how the WMS GetCapabilities request can be used to create an automated report listing all the map layers available from a WMS server. The final sample page can be seen here.

The GetCapabilities Request

Web Mapping Services (WMS) is an open standard that all major GIS vendors implement in their server software (for example see the ESRI documentation). As a result applications built on these services are able to easily switch from one software package to another – allowing separation of code that allows for easier maintenance and reuse.

The GetCapabilities is one of three requests in the WMS specification. It “returns service-level metadata, which is a description of the service’s information content and acceptable request parameters.”

After eventually being able to see the results of a GetCapabilities request my aim was to generate a report listing the available map layers, and to check I had enetered all the required metadata. I had a look on the web to see if there were any stylesheets that would nicely format the data.

Continue reading »

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