Archive for the ‘web development’ Category
OpenSource Upgrades
Moving to new versions in OpenSource has in the past been like playing Russian roulette. Just now clicking on “Upgrade to WordPress 3.0″ I thought would lead to a greater understanding of the database schema, template inconsistencies, and plug-in idiosynchrocies of WordPress as I met an inevitable “Upgrade Failed. Please see log for details” message.
But no! Five seconds later, all was successful, and everything looked as before. There have been numerous bug fixes and improvements, but after a brief look through the list I didn’t see any “killer” new features.
In the world of GIS however there are a number of important new versions coming up this summer.
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We Know Where Your Browser Lives
Maybe I’m a little late in finding this, but if you have FireFox 3.5 or higher try clicking this link (you will have to agree to let your browser divulge your location to open the link). The JavaScript code run when clicking the link is as follows, and can also be pasted directly into your address bar:
javascript:navigator.geolocation.getCurrentPosition(
function(position){
window.location.href="http://maps.google.com/?ll="+position.coords.latitude+",
"+position.coords.longitude;});
I believe the navigator.geolocation object is also available in Safari, iPhone, and Chrome. The Mozilla documentation provides more details on its use. This StackOverflow question asks which browsers and devices it applies to, and this is set to grow if it becomes part of the browser standards – the W3C has an online draft Geolocation API Specification. They list the following use cases: Read the rest of this entry »
Visualising GeoJSON in 15 Minutes
The objective of this post is to save a GeoJSON feature collected from the web to an image file, and was inspired by the Shapely manual. This will be done using Windows, Python 2.5 (although 2.6 should be almost exactly the same steps), and some additional Python packages. This example uses GeoJSON data from a CloudMade geocoding service.
The greatest difficulty when trying to do things in Python is getting the right packages to match your Python version, and making sure all the package dependencies are in place.
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GeoJSON Where are You?
I was looking around the web to try and find some services that returned GeoJSON to try out Shapely. It was surprisingly hard to find servers or web services given the amount of tools that have already been built to use GeoJSON.
Eventually I found the CloudMade’s geocoding developer samples. CloudMade is a company that builds tools and services on top of OpenStreetMap. Even though many of the people involved in both CloudMade and OSM are the same, using the two brands interchangeably has not been without controversy.
Once you’ve signed up for a free account, you can then register for an API key that allows you to call their web services. See here for differences between free and paid accounts. The service I was interested in testing was the geocoding service that returns GeoJSON.
Depending on the type of feature you are looking for you can get different geometry types – the standard point, lines, and polygons, all as GeoJSON objects.
The API uses URLs and GET requests to return data. Calls can be made to either a find.html page which returns the GeoJSON object displayed on a map, as shown in the query for “england” on the left. Alternatively the query can be made to find.geojs to return just the GeoJSON itself. There is also a find.js which can be used to return geometry as JSON, although this seems a little confusing – even more so as the coordinate order is reversed from the standard GeoJSON [lon,lat].
Watch out for OpenLayer Distances
I 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. Read the rest of this entry »
Compiling a 64 bit Version of PyISAPIe
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!”…
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Logging OpenLayers with Firebug
I 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.
