Invest in Accuracy

I spend a lot of my time working with maps and plans for various clients and most of my work is undertaken using mapping software. Digital mapping continues to advance at rapid pace and over the past few years we have also seen a lot of 3D data becoming available. One data supplier has mapped the whole of London in 3D and that is a very impressive product. Whilst I do work with 3D data the majority of my time is spent using 2D and I have worked with everything from 18th century maps through to currrent day products. However whether 2D or 3D one aspect remains key - accuracy.

Choosing the right map

Over the last 100 years accuracy of maps has improved significantly after all we have gone from the measuring chain to satellites over that period. As such when we purchase map data for whatever task we are undertaking we can be fairly certain that the map has a sufficient degree of accuracy for its intended use (more on that later). The primary source of map data for the UK is the Ordnance Survey and they maintain the national map data sets and keep them upto date with over 10,000 updates made to the database every day. I would still like to see the cost of map data reduce significantly but that is for another blog.

Choosing the right scale

Selecting the correct scale of map for the task or job in hand is key. If for example you are going walking for the day you wouldn't wish to use 1:2500 scale maps are they would simply be too big and not appropriate when the user needs to see the wider landscape. You would select something like the OS Explorer maps. Conversely if you were having to identify a parcel of land the Explorer map is of little use.

Choosing raster or vector maps

An additional consideration is whether to use a raster or vector based map. In short a raster map is an image made up of millions of pixels which can be used as a back drop. Whilst a raster image can be rich in colour and detail it looses quality as you zoom it and you can't manipulate it. Typical uses would be a backdrop to additional information you wish to display, for example supermarkets within 25 miles. A vector map is made up of points, lines and polygons which collectively make up all the features on the map. In addition you can add additional data to these features. For example you could record details of the age and condition of buildings and then highlight all the buildings that are in "poor" condition.

Common Errors

During my work over the past 30 years the most common error has been using the wrong base map, whether that is due to scale or format. How many times as surveyors have we used last years plan for a new grazing (or similar) licence, photocopied it and marked the area edged red with a thick line which in the real world is probably about 30 metres wide. That plan may give you a good indication of where the land parcel is and for a simple licence it may have been fit  for purpose. However the same approach is often used when buying and selling land and in this case errors can be costly.

Adding Data to you map

It follows that if you have selected the wrong scale of map then your data is likely to be displayed incorrectly or with sufficient accuracy so lets assume you now have the correct base map.

Firstly you need to consider the source of information you are adding to your map. This could range from your own knowldege, eg defining the area of that grazing licence to importing data from a third party which could be those supermarkets. 

Looking at detailed mapping first a common task for surveyors is identifying parcels of land or other features like a right of way or easement. A parcel of land is most likely to follow field boundaries so following that boundary should be straighforward. If you are using a raster backdrop with care a line can be drawn to represent the area. If using vector maps a bit more care is required to ensure that your new  property boundary is exactly the same as the vector map. Common issues here are gaps being left between parcels that do have a common boundary. It is more difficult when the feature you wish to map doesn't follow field boundaries or other map features, examples here are easements, power lines, pipelines or say a new fence line. I will cover these in another blog.

If you are working with 3rd party data be very careful and understand it's origin and how it was mapped and to what scale. An example here could be downloading the geographic extent of a local authority. The author may have mapped the polygon at 1:25,000 scale whereas you may want to look at a feature and where it is relative to that boundary and need 1:2,500 scale. In this exampole the line may not be sufficiently accurate. Another good example would be the supermarkets mentioned above. You can buy data sets of shops, garages, supermarkets etc but these are normally mapped using their postcode. In simple terms the point or location on the map will at the centre of the postcode it is within. The point shown on your map could be a few hundred metres from where it actually is.

This blog only highlights some of the more common issues realting to accuracy of maps and data. You will find that using the correct base map for the task does cost more in the short term but if you map the data you want to capture accurately you will only have to map it once.  Furthermore if you are working with mapping software who will be able to use that data either on its own of combined with other spatial data you have and this is where GIS can be a very poweful tool.

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