WebGIS application with OpenLayers 2 - Part 3: Data management

In the last part of the series, we will cover data management in OpenLayers 2. It is a quite difficult part, as browsers mostly don't have native support for relational databases. The only exceptions are browsers powered by WebKit. This way, we have to make sure manually, that our data is stored in a consistent way. ASCII geodata formats, which are mainly used in web mapping environments don't support databases. We can store different attributes for every geometry in a layer/dataset. This doesn't seem to be an issue on the first glance, as web mapping libraries can handle object-oriented databases very well. However, if you declare attributes arbitrary, in an inconsistent way, you will have a hard time when you try to process it with a RDBMS. As the server side processing GIS softwares (e.g. QGIS, GRASS, PostGIS) will put these datasets into a relational database, it might be a wise consideration to handle features like they were in a relational table on the frontend.

WebGIS application with OpenLayers 2 - Part 2: Map controls

So far we have created the basic container for a web mapping application GUI. The key aspect in a good application is user experience. This mostly depends on how easily a user can utilize the application, and how smoothly can accomplish a workflow. For a decent user experience, one should think as a user when designing the appearance and mechanics of the application. To make things go right in a WebGIS application powered by OpenLayers 2, the library offers a wide variety of controls. As developers, the responsibility is on us, if we get the most out of it, or not. This is the part where OpenLayers 2 shows its advantages even against its successor. The following post will demonstrate how we can extend our application with minimal programming, and mostly controls offered by OpenLayers 2.

WebGIS application with OpenLayers 2 - Part 1: Layer tree

Creating a whole WebGIS application is a time-consuming task, which can be eased with toolkits, like GeoExt or Heron MC. However, using such a toolkit, has some disadvantages. For example, you can't keep step with the evolution of the mapping libraries, as the toolkits can't develop so rapidly. If you want to use the most recent features, and don't want to wait for the developers of your favourite toolkit to come up with a new version, you have to build your application just on the library.
From another point of view, learning how a web mapping library works can be rewarding. If you build your WebGIS application, you can easily come across a situation, where you have to customize it in a way, that hasn't been implemented in your toolkit. At this point, you will need to learn how to use the native library anyway (and consider yourself lucky, if you don't work with a deadline). To avoid such a case, it is considerable to get acquainted with the library in time and use toolkits later. In the next few posts, I will demonstrate how to build a basic WebGIS application with only HTML, JavaScript, and OpenLayers 2. I will try to summarize the content of the last few posts, and show how to handle browser events with the library.

Controls and projections in OpenLayers

OpenLayers has a huge advantage as a Web GIS GUI over Leaflet. It can handle any projection, which can be described by Proj.4. Leaflet supports only two projections (the WGS84 and the Web Mercator). These projections are natively supported in most of the GUIs. As Leaflet is a plugin based, modular library, it has a plugin to use Proj.4 capable projections, but it makes a reverse transformation. It transforms the input coordinates with Proj4js from the defined projection to geographic coordinates. OpenLayers on the other side is capable to handle any coordinate system, so the input maps will be rendered in a projection defined by the developer.

OpenLayers layer management

In the previous post I wrote about layer creation. This is only a part of the capabilities of a good GIS application. A good application can also manage layers in an efficient way. Fortunately, OpenLayers libraries have predefined functions for layer management. They provide a great base to build upon and create dynamic Web GIS applications. Some of the basic tasks of layer management are adding and removing layers, changing layer order, listing rendered layers, and changing layer styles. Off course, they have to do it during the workflow, without reloading the page. I will show a few of these functions with a live example in this post.

OpenLayers layer creation

Layers are a core concept in every GIS software. They are collections of contiguous spatial data. You can stack layers on each other, perform an operation on them, style them individually, etc. Fortunately, OpenLayers is outstanding with its layer management. Every raster dataset is contained in a layer. For vector data, you can group features to as many layers as you like. Layers can be accessed, modified or removed after they are constructed. As the layer changes, the map object will track the changes, so it will take effect on the visualization, too. In the following post, I will write about OpenLayers layers.

Getting started with OpenLayers

In the world of open source web mapping interfaces, the biggest and most robust one is OpenLayers. OpenLayers was first released in 2006 by MetaCarta Labs. After 2007, it became an OSGeo project maintained by the OpenLayers Developer Team. It is still under development in these days. The two well-known and widely used branches of this project are OpenLayers 2 and OpenLayers 3. The third version was officially released this year (August, 2014.), and OpenLayers 3.1 has been announced. This also means, that OpenLayers 2 got out of development and the current version is its final form. OpenLayers 3 on the other hand is under constant development, so applications written with it may require frequent maintenance. In return we will get an interface armed with the capability of using up-to-date formats (e.g. TopoJSON), having beautifully rendered maps (WebGL) or in the future providing 3D maps (based on Cesium).