Thursday, 2 September 2010

Scale matters when using digital maps for surveys

Misunderstanding the meaning of scale and its application to surveys can result in products which don't meet anticipated needs. Scale is important because it defines overall content and accuracy of a survey in a very basic way.

Fundamentally scale is not absolutely related to accuracy but in a graphical product it will limit the accuracy that can be achieved.  For example, 1:1 means what it says; a survey which is life size. It does not mean ‘dimensioned in metres rather than drawing millimetres’. For example, 1:100 means that an object 1m long on the ground will be 10mm long on the plan. When talking about relative scale size, a scale of 1:50 is larger than 1:100 and would require a larger hardcopy plan to represent the same physical object or area.

Understanding how scale are used in surveys means producing the required amount of detail to the desired accuracy, without needing to understand all the processes involved. Of course, if you are familiar with specifying surveys then a full specification will get the best for your project, but if that is not your favourite pastime then getting the scale right is a good second best. However, remember for unmapped territory you need a full Chartered Surveyor.

So where do we start? First of all, what accuracy do we need? Traditionally, surveyors will show detail correct to within 0.2mm at the plotted scale. So, if you want detail to be accurate to within 50mm, you should specify a plot scale of 1:250. But don’t forget that this is a guide. It does not apply to underground services and if the accuracy of any feature is critical you should tell the surveyor. Secondly, what detail needs to be shown (resolution)? Normally we show any point objects to scale if they are larger than 1mm at the plotted scale.

Read more on the RICS website about scale
View, print and export maps at different scales on FIND

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