|Posted by Jonathan Gatward Huddersfield Yorkshire on November 6, 2012 at 1:00 PM|
For most of use, when we need to know where something is, we look at a map, click on Google or tap into Sat Nav. We can find out where we are, where we need to be and how far we have to go. This usually gives us the information we need to know at sufficient detail and accuracy for what we need.
But..Is this information always correct ? Does it always give you the right answer ? Is it accurate enough ?. Well the short answer is "No", the slightly longer answer is "It depends what you want to do with it" and the full answer is... well read on!
Maps come in different scales, designs and formats, each with its own characteristics, flaws and advantages. The creation of a well designed map is not a wholly automated process and involves a lot of skill and experience to produce something that is clear to read, provides relevant information, is accurate and looks good. This is where the cartography aspect of map production comes in.
Every map has some design compromises, how big are the symbols so they can be seen but not become too cluttered?, which information do we leave out? how detailed do we show the roads ? are they represented as a single line of a certain width, or do we show the actual edges of the roads? do we include pavements or is this shown seperatley ? These type of decisions are always made in map production, either consiously or as a 'brain stem' function of 'making a map look good'.
Here lies the problem though, of representing real world features on a piece of paper of limited size.
At most map scales (eg. 1:10,000 and above), to be able to see a real world feature such as a narrow road, its representation or symbolisation on the map needs to be made bigger than it is in real life. For example, a narrow track may not be much more than 2 metres wide, which on a 1:10,000 scale map, would be represented at real size as a line of 0.2 mm thickness, way too thin to see it as a road. As part of the cartographic design process, the road is ‘thickened’ up to a line width of 2mm (equivalent to 20 metres in the real world).
So now we have a road on the map that is perhaps 10 times wider than in real life. “So what?” you may think, but what happens to the representation of buildings that lie adjacentto the road? If the road is shown wider than it actually is, buildings shown in their correct location will show the road running right through the building, which will just look messy, confusing and silly. So again, cartographic design comes in to play and the buildings are moved, simplified and generalised so that they appear in the correct ‘relative’ location to the road but they are in fact in the wrong place.
This is not an issue if you just want to find where the road goes to, but if you want to measure distances from the edge of the road or from the buildings that are next to it to some other feature, then you will be getting a wrong measurement. Whether the extent to which the measurement is wrong is an issue or not again depends for what purpose you are taking the measurements, but certainly if you are concerned with measurements with an accuracy of greater than +/- 20m, then you really need to be thinking again and possibly using larger scale mapping (eg. OSMastermap).
So, don’t take what is on the map as definitive – it is only as good as the scale and purpose of the map that it is shown on and the cartographic decisions that were made by the map maker. It is an aspect of all maps that they are a simplification of reality and part of this simplification is to show real work objects slightly changed from their real work locations and forms: that is objects shown on a map are actually likley to be somewhere else!