OpenLayers and Versioning in Aptana Studio

Jun 302011

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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Continue reading »

MapServer, OpenLayers and the WFS Maze

Jun 102011

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. Continue reading »

Accessing Cross Domain Data with YQL

Apr 102011

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. Continue reading »

Will HTML5 Revolutionise Raster Web GIS?

Feb 182011

image Or perhaps that should be rephrased start browser-based raster GIS?

GIS data is split into two base types – vector data – geometric shapes, usually further split into points, lines, and polygons, and raster data – cell-based or “pixelated” data.

Graphics on the web mirror this divide. On the vector side SVG – scalable vector graphics, is used in many browsers to display geometric shapes. On the raster side “dumb” images come in many well known formats such as bitmaps, GIFs, PNGs, and JPEGs.

Vector geometries are easily manipulated after drawing as they have an abstract model to work with (the SVG, or KML document), which the browser can then convert to the DOM. As an example OpenLayers includes two vector renderers – one for SVG (see source code), one for VML (used by the ever-unique IE), and since the start of this year a new canvas renderer.

The canvas renderer is used to draw features to the new canvas element which is part of the HTML5 specification. This allows access to images loaded into the canvas through new programming interfaces such as the Canvas 2D Context API.  It is this part of the HTML5 specification that could change the way we work with raster data on the web.

A Proxy for IIS and .NET

Dec 072010

Proxy ServerDue to the same origin policy any data from a remote server cannot be (easily) added to a web application on your own server. This issue also applies to WFS (Web Feature Services) and OpenLayers. There is a Python script that can be used to get round this issue, but I preferred to have a native .NET equivalent.

On the OpenLayers Mailing List Diego Guidi pointed to an opensource .NET proxy. A proxy makes a request to a remote URL, reads the response, and then sends it to the client so it appears all data comes from the same server.

The .NET proxy is written by Paul Johnston, and can be found at http://code.google.com/p/iisproxy/. I made a few minor changes as follows:

  • increased the byte size for reading responses to resolve this issue
  • added support for PUT requests
  • temporarily removed GZIP compression due to invalid responses and a possible bug in .NET

The proxy can be used for any requests and is not limited to just OpenLayers. My source files can be found at http://bitbucket.org/geographika/openlayers/src/bfeab6a9971a/iisproxy/

I’d recommend reading the original project’s README file which goes through compilation and installation, but I’ve added my own notes below. Continue reading »

Creating a Geodesic Circle in OpenLayers

Nov 272010

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I recently asked a question on GIS Stack Exchange on how to create a  buffer around a point that took into account the curvature of the earth. OpenLayers has support for geodesic measurements, but not creating geodesic polygons. Drawing a standard polygon on a Mercator projected map can produce features with very different measurements from their intended “on the ground” equivalents.

Paul Ramsey pointed out that “the scale errors for Mercator are very high indeed (infinite, in fact, at the poles) increasing as you head north/south from the true scale latitude.” In fact drawing a circle (in Ireland – 53 degrees North) with a 10km geometric radius produced a circle with an on the ground radius of 6km.  A huge margin of error over a very short distance (see a previous post on the same subject).

After a useful answer from Dan Shoutis, it appeared most of the work to implement geodesic circles was already available in the OpenLayers API. The OpenLayers’ geodesic functions are based on code adapted from Chris Veness work at http://www.movable-type.co.uk/scripts/latlong-vincenty-direct.html.

The code to create a regular (non-geodesic) polygon can be seen in the OpenLayers source here. There is a function added after the class OpenLayers.Geometry.Polygon.createRegularPolygon that can be used to “create a regular polygon around a radius. Useful for creating circles and the like.”

This function only required a couple of changes – notably using Longitude and Latitude and rather than X and Ys, to produce geodesic polygons. If you are using the Mercator projection then transformations requires proj4js support. Continue reading »