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| | | 3.4 Technical Analysis of OpenLayers | Contents | |
At this point the functionality of OpenLayers will be examined in more detail. The purpose of this analysis is to increase the understanding of the application as well as to serve as a premise for the subsequent conceptualization and implementation.
OpenLayers is a pure JavaScript API for the creation of web mapping applications; it also offers support for the use of numerous (standardized) formats (OGC WMS, OGC WFS, GeoRSS, ka-Map, WorldWind and many more) which are integrated as layers into OpenLayers.
In addition, OpenLayers features panning and zooming of maps, client-side tiling, markers, »popup windows«, various adjustable navigation components, keyboard commands, a robust event handling mechanism and more. Each part of OpenLayers is configurable [OpenLayers d].
A Python/Shell-based script enables the automatic creation of a lite version of OpenLayers, whereby selected classes can be integrated into one single JavaScript file [OpenLayers a]. This is a result of a discussion that took place in September 2006 at the FOSS4G Conference, where such a solution called webmap.js was requested [OSGeo 2006a][OSGeo 2006b].
The target group for OpenLayers includes all users or developers who wish to display a map on the Internet, or build a map-based Internet application [OpenLayers d].
The February 2005 release of Google Maps drew a lot of attention. As a result of numerous reverse engineerings of Google Maps codes (by Phil Lindsay, among others25) Google responded with its first Google Maps API26 in June 2005. However, the possibility for commercial implementation did not exist [Ramsey 2006][OpenLayers f].
Recognizing this deficiency, the US company MetaCarta, in cooperation with Phil Lindsay, developed a first prototype of the eventual OpenLayers which was presented in late June 2005 at the Where 2.0 Conference in San Francisco27. One year later, shortly after the June 2006 Where 2.0 in San Jose, the team headed by developers Schuyler Erle, Chrisopher Schmidt and Erik Uzureau released the first official version (1.0) of OpenLayers on July 5, 2006. Not long after, in August 2006, Version 2.0 of the project was released[OpenLayers d][OpenLayers f].
At present OpenLayers is undergoing the incubation process at the independent Open Source Geospatial Foundation (OSGeo), with the goal of being accepted as a software project by the summer of 2007. Parallel to this process, serious efforts are being made to integrate key functions of OpenLayers into other Free web mapping applications [OpenLayers d] - with ongoing discussions at ka-Map28,29, Mapbuilder30 and Mapbender31.
Because of its object-oriented programming, the API of OpenLayers is divided into a multitude of classes, of which the main ones are briefly described below. The remaining classes are not discussed in any further detail, as they are not a requirement for a basic understanding of API. More detailed information can be found in the JSDoc documentation32. A complete classification diagram of OpenLayers (Version 2.4 RC5; as of May 25, 2007) can be found in Appendix 6.3.
Below four layer classes are described in more detail:
OpenLayers.Layer.Grid
Derived from the HTTPRequest class, Grid serves as one
of the important primary classes for the above-mentioned Free
tile-based layers. Grid.js divides the layers into tiles,
holds them in an array, and loads them as a spiral-like sequence
beginning from the middle.
OpenLayers.Layer.WMS
As a subclass of Grid, WMS receives the URL of the
WMS servers and is responsible for generating the individual tile WMS
URL's based on the desired tile boundaries (bounds).
OpenLayers.Layer.WMS.Untiled
The Untiled class displays the complete map section in a WMS
tile URL. Note that many WMS servers will have a pre-defined maximum
tile size of 2048 pixels; if OpenLayers requests a larger map
section, the WMS server will send an error message and the map view
will remain empty.
OpenLayers.Layer.Image
Derived from Layer, the Image class permits the
display of individual bitmap graphics as map layers. Besides a URL, the
pixel size as well as the geographic dimension of the requested picture
are also required.
OpenLayers.Control.PanZoom
generates pan/zoom navigation with a panning pad (4 arrow buttons), as
well as a ZoomIn, ZoomOut, and ZoomReset button. The
slideFactor parameter defines the number of pixels used
during the panning processes.
OpenLayers.Control.PanZoomBar
generates a pan/zoom bar with a pan navigation panel as well
as a zoombar with a zoom slider containing a ZoomIn and ZoomOut button
at each end. (see Section 2.3.4).
The zoom bar is integral to the development of animated
zooming features. For this reason its functionality is described in
more detail.
There are three ways to use the zoom bar:
OpenLayers.Control.OverviewMap
generates a small overview map
(see Section 2.3.4). This map shows the current view of
the main map and serves as a positioning and navigational tool. The
overview map is usually located in the lower right hand side of the map
and can be minimized by pressing a button on the map edge. This class
provides the functions needed for querying and setting of the selection
rectangle, which can be moved by way of defined mouse events.
OpenLayers.Control.KeyboardDefaults
defines type and activities of (keyboard) key commands.
OpenLayers.Control.MouseDefaults
defines map activity during mouse events.
This includes click, double-click, mouse wheel and mouse movement events.
OpenLayers.Tile.Image
Derived from the Tile class, the Tile.Image object
presents the proper tile graphic and creates an img-Div-HTML element for
each tile during the drawing of the map.
OpenLayers uses the Test.AnotherWay33 framework for unit tests. Version 2.4 RC5 of OpenLayers (as of May 25, 2007) supplies almost 1500 unit tests, which are controlled over a central web interface34. The framework offers the ability to test HTML and JavaScript source codes, and also provides the test results.
Each HTML test page contains one or more JavaScript test functions which start with test_; they must also define a test object t as the transfer parameter [Test.AnotherWay][OpenLayers g].
Listing: A sample of a simple test page with the a function
<html>
<head>
<script src = "../lib/OpenLayers.js"></script>
<script type = "text/javascript">
function test_Map_Zoom(t)
t.plan(1);
var map = new OpenLayers.Map("map");
var layer = new OpenLayers.Layer.WMS("ABC",
"http://example.com/123",
'layers':'test');
map.addLayer(layer);
map.zoomTo(0);
t.eq(map.zoom, 0, "Map zoomed to level 0 correctly.");
</script>
</head>
<body>
<div id="map" style="width: 512px; height: 512px;"/>
</body>
</html>
Listing 3.4.3 shows a sample test page which can be used for a simple testing of an OpenLayers function. On line 6 the test function defines the number of planned tests (here: one), with the expectation that they will be completed correctly. If the number does not match the number of actual tests, the test function is not successful. Lines 7 to 12 produce a map with a WMS layer, at zoom level 0. Line 13 tests the assertion that the map is actually at zoom level 0.
The testing framework provides five methods for the testing of self-defined assertions:
Examples and additional information on the functionality of the framework can be found in the [Test.AnotherWay] documentation.
© June 1, 2007 |
Emanuel Schütze |
some rights reserved.
This work is licensed under the Creative Commons License
Attribution-ShareAlike 2.0 Germany.
| | | 3.4 Technical Analysis of OpenLayers | Contents | |