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14th Jan 2010
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TrackLogs Digital MappingTrackLogs Digital Mapping

How accurate are the maps?

TrackLogs maps use digital data licensed from the Ordnance Survey, inheriting their worldwide reputation for accuracy. As a general rule, larger scale maps offer the highest accuracy. TrackLogs products in order from highest to lowest accuracy are 1:25K Explorer, 1:50K Landranger, 1:250K Roadmap, 1:1M Miniscale.

To be more specific we need to understand what is meant by accuracy.

Absolute vs Relative Accuracy

Two of the most useful accuracy metrics are 'absolute' and 'relative':

  •  
Absolute
Absolute accuracy is the position of features in relation to the Ordnance Survey National Grid. In other words it's how precisely an object's position on the map corresponds to its position in the real world.
  •  
Relative
Relative accuracy is a measure of how much detail is drawn on the map and how realistically the objects relative positions are depicted.

This distinction matters because how we use maps in navigation is changing. Consider a walker approaching a track junction. A printed map can be used to establish which course to take and this decision is made by looking at the information drawn on the map. More map detail is good and the relative positions of the objects must be correct but absolute accuracy is less important.

Now consider the same walker approaching the same junction only this time the printed map is replaced by a GPS device. If the walker is following a pre-determined trail it's critical that the absolute position of the track junction on the ground matches the position in the GPS. This demands high absolute accuracy and de-emphasises any relative accuracy or side detail.

TrackLogs Digital Mapping

Accuracy In Action.

We've taken a GPS recorded track and overlayed it onto a digital map. The majority of the image is using 1:25,000 scale Explorer mapping and, at this scale, the GPS recorded track shows good correlation with the marked bridleway. To compare this with 1:50,000 scale Landranger we've inserted an area of 1:50,000 scale mapping in the centre. At first glance the relative accuracy is similar. The contours, paths and wooded areas can be seen in either mapping scale and these loosely tie up at the boundaries. On closer inspection the 1:25,000 Explorer does include some additional detail, specifically field boundaries and open access areas. This is an improvement in relative accuracy or map detail.

A more significant observation concerns the difference in absolute accuracy. Within the central 1:50,000 scale region the GPS track is seen to deviate from the marked bridleway. This loss of absolute accuracy would most likely go unnoticed when navigating from a printed map, when following a GPS track this deviation could be significant.

For detailed navigation work we always recommend 1:25,000 scale Explorer maps as these provide the necessary absolute accuracy to maintain a good lock between GPS position and map features. 1:50,000 scale Landranger maps are the next best and these can also be used successfully with GPS. In all cases you should backup your GPS navigation with a TrackLogs printed map.

Written by TrackLogs Digital Mapping. Copyright TrackLogs Digital Mapping 2002-2017. All rights reserved. Please send any comments, questions or feedback to support@tracklogs.co.uk


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