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.

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).

Web mapping interfaces

When you start to develop your web GIS application, the first consideration will be the the interface you wish to work with. There is a wide range of open source GUIs. There are a lot of similarities in them, like they are written in JavaScript (the library which you will use to publish your maps if not the whole software), they use similar sets of values, they use similar methods to reach servers or handle geospatial data. There are lot of comparisons between various GUIs, but in reality there isn't such a thing like best and worst interfaces. Well, maybe there are worse ones, but I won't write about all of them. I will show some of the most popular ones, without attempting to be comprehensive. This post will be a prelude to using these interfaces, showing general considerations and methods to use them.

Map server basics

If you would like to build your own map server, you have to think it through carefully first. Do you need a complete map server or you can stack with an existing one? Is Google Maps good for you or you need more control on your maps? There are several options on the internet and with some of them, you just have to write a brief JavaScript code and voilà, you have a well-visualized map. In the next few articles I will show some options which won't need an independent map server, but learning at least one of them will be inevitable if you have to build one.