gvSIG blog

One of the small but important improvements that have been included in gvSIG 2.0 is, in our opinion, the access to the excellent data source that OpenStreetMap is. The extension, that was recently released, can be installed easily from the gvSIG add-ons manager (by URL, selecting the http://downloads.gvsig.org/download/gvsig-desktop/ server and the Formats: OpenStreetMap raster tiles support) package.

As it was commented in a previous post, it will allow us to access to Map Quest, Map Quest Open Aerial, Open cycle Map and Mapnik layers.

An interesting functionality that is presented in this extension is that the user can add new servers.

For that we only have to fill in the data of the new server in the Add layer/OSM window.

We’re going to see an example where we’ll add Terrai, one of the Stamen tiles services, from United States of America, all of them with an excellent design.

• Name: We can add whatever we want, a name that allow us to identify the server.
• URL: The URL of the server: http://a.tile.stamen.com/terrain
• Levels: In this case they are 17
• Format: In this case it’s jpg

It would be something similar to this image:

We only would have to press the “Add server” button to include it in our server catalogue.

Of all the servers that Stamen offers, the server that we like the most is the Water Color one. We’re going to add it:

• Name: Stamen Water
• URL: http://tile.stamen.com/watercolor
• Levels: 17
• Format: jpg

We only have to press “Add server” and it would be added to the group of available servers.

You can check to view it. You have to take into account that not all of these servers offer information of all the world, different from the 4 servers by default. In the cases where Stamen doesn’t cover all the world you can zoom to a city of USA and check how the new information is shown.

gvSIG 2.0 improves the raster access incorporating the possibility to make tiles for the sources of this type of data, local or remote ones. The results is that the raster layers are loaded on the gvSIG view in small portions or tiles that allows the layer to be rendered progressively. The layers are divided in pre-establish levels with different resolutions and these tiles are generated for each level in executing time, that means when the requests for new areas are made (zooms) the tiles are generated.

But the best advantage of this system is that we can save all the tiles in a cache. At this way, the information that is downloaded from the servers don’t have to be asked for again once it has been done, because the next requests are served from the cached data, with the consequent efficiency improvement.

At this way, the incorporation of new data sources with this system of resolution pyramids is easy. Due to this possibility the development of a new plugin to access to the tiled layers from OpenStreetMap servers has been relatively easy. It’s because these servers serve data at this way and it’s totally compatible with the cache system that has been incorporated in gvSIG.

This plugin includes a basic access to the OpenStreetMap layers for viewing. Four servers are configured by default, that serve the Map Quest, Map Quest Open Aerial, Open cycle Map and Mapnik layers. At the same “Add layer” interface it’s possible to add new servers, so it doesn’t limit it only to the preconfigured data. The interface to do it is very intuitive and it doesn’t need any special explanation. In our opinion, this is an example about how we can take advantage of the base possibilities of gvSIG 2.0 effortlessly to develop a new functionality.

A problem with the OSM tiled layers in gvSIG is the projection. They are served in EPSG:3785 and gvSIG doesn’t include this projection originally. It’s possible to create it manually if we have the WKT string, but it’s not easy. In this case, the plugin will do it for us. Everytime it’s executed it will check if this EPSG exists in the gvSIG database and if it’s not included it will be created as a user projection. At this way we will be able to change the view to this projection for loading the cartography.

Another inconvenient that we can find is that once the information is at the cache it’s not updated, and if the information changes at the server we will continue loading old data. This is not a problem to load local layers, but it would be a problem for remote layers, like the OSM layers. We expect to improve it in the future to solve this problem.

In the previous posts of the symbols series, we have seen how to create a symbols library form a set of images, realising some examples such as Google and OSM libraries.

It can be interesting as well to realize our own symbols library from different external sources. They can be made of letters, numbers or special characters as symbols or even graphic symbols.

As an example, we are going to use one source containing animal symbols.

After having downloaded it and installed in our system, we will transform it into images by using Inkscape, an open source software for vector editing that we have already mentioned in other posts.

To install a source it is only needed to open it with the typographic viewer of our operating system  and click on Install.

Once installed it will be shown as part of the available fonts in Inkscape. Add text and, for example, if we write “ABCDEFGHI”, something like this will appear:

We will transform these characters into single images in two simple steps:

• Path/Object to Path: transform the character into image.
• Object/Ungroup: create a single image for each character.

Then we have just to select each of them and save one by one as single file by “File/New”, assigning to any single document a size similar to the image ones. Cut and paste the image into this new document (for example the first, the caiman) and by “Object/Align and distribute” align it in the centre. Save it with a name easy to be identified (Caiman.svg). Done.

To have a different symbol when selected, fill the image with yellow colour and “Save as” with the same name but adding at the end “_sel”.

This action can be repeated to as many symbols we want to load. From now on, we only need to use the gvSIG symbol loader in order to have our new symbols library available in gvSIG.