[GIS] How accurate is the Scale Line in OpenLayers

openlayers-2scale bar

Is there any source describing the accuracy of OpenLayer's scale line?
You can get an scale line by using the constructor:

map.addControl(new OpenLayers.Control.ScaleLine());

However, it seems to be very inaccurate in relation to my geodesic measurements (0.5 km – 1.5 km difference depending on zoom). Is there any info I can cite to explain this different?

I'm using OpenLayers "Release 2.13.1" and OSM as base layer. I'm calculating the distance between point using WGS84 projection with 3 decimals and geopy distance module.

As an example, the distance between the points -2.416,55.703 and -2.434,55.698 is 2066.599 m according to my calculations and 2.5 – 3 km according to the scale line depending on zoom.

Ideally, the info I'm looking for is something like "the scale is accurate in +- 1 km because it calculates the distance using X method". The official doc (http://dev.openlayers.org/docs/files/OpenLayers/Control/ScaleLine-js.html) doesn't mention accuracy.

Best Answer

The ScaleLine represents the scale of the map on the screen, and hence is intrinsically different from the scale of a printed map.

The OSM tileset is built using the Web Mercator projection, and this projection does not maintain scale. This is why you do not get perfect scale.

If you want the geodesic scale, you should use the geodesic:true parameter while constructing the Scale line control. You should note, that this gives the geodesic scale at the center of the map, which might not be the same as the scale at other parts of the map.

Related Solutions

Geodesic Measurements – Using for Circle Radii in OpenLayers

If you're home-brewing in the browser, you can get a "circle" (it will not be round on the screen due to your projection; rather approximated by a polygon w/ as many points as you care to draw), use a the direct form of geodesic calculations: given a point, a direction (azimuth), and a distance it gives you the resulting point. Gory details: http://en.wikipedia.org/wiki/Vincenty%27s_formulae#Direct_Method

Looks like someone has done a translation to JavaScript already: http://www.movable-type.co.uk/scripts/latlong-vincenty-direct.html. Lucky you!

To finish things off:

Decide how chunky (# of vertices, call it n) you are willing to have the end result be.
Divide 360 degrees into n pieces.
Build a polygon by (for i in range(n): polygon.add(vincenty_direct(start_point, i * 360/n, distance)))
After-the-fact, possibly fix up some projection and planarisation irritations:
- If you're using the typical web map projection, which you almost certainly are, the resulting polygon will be hugely stretched vertically if it nears a pole.
- Similarly, if the result polygon crosses the international date line, it'll be really borked.

[GIS] Finding URL of GeoServer layer

The short answer is

http://localhost:8080/geoserver/cite/wms

The longer answer is that the URL in OpenLayers is the base URL of the web map server (so all the bit before the ? in the full URL). OpenLayers then adds the required parameters to make up the URL that is required to get a map. Some of those parameters are added from your layer definition (i.e. layers, transparent etc) and others from the map object (i.e. width/height and bounding box). See Lesson 8 of my old course to see a fuller discussion.

Best Answer

Related Solutions

Geodesic Measurements – Using for Circle Radii in OpenLayers

[GIS] Finding URL of GeoServer layer

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