Tolerances adopted by RoS when using the OS Map
A whole range of answers can be given to the simple question “How accurate is the map I’m looking at?”.
The accuracy of the OS map is influenced by such things as:
survey scale
survey methods
cartographic methods and practices
age of map data.
In addition, what do we actually mean by accuracy - is it absolute accuracy we’re talking about, relative accuracy, or geometric accuracy/fidelity? All of these terms are covered in a bit more detail on the guidance page Accuracy of the OS map, and the information on that page will be helpful if you need to understand in more depth the statistics and what to expect on the various measures of accuracy of the map.
Tolerance
However, on a day to day basis, what is probably most useful to a plans officer is an understanding of the tolerances within which RoS should work when plotting onto the OS basemap. Tolerance can be thought of as the amount of leeway or difference in a measurement that is acceptable at the different base scales.
RoS primarily works to the principle that the acceptable tolerance in the measurement or location of a feature or point is within the following distances at each survey scale:
1: 1,250 - up to 1 metre | 1: 2,500 - up to 2 metres | 1: 10,000 - up to 8 metres |
These figures for maximum acceptable tolerance have been arrived at by taking into account the various different historical survey techniques and accuracy standards at the different basemap scales, with help from the OS.
We apply these tolerance rules when comparing a feature shown on the deed plan with a feature shown on the OS basemap - for instance, does the length of a feature shown on the OS map coincide with the length of a legal boundary shown on the deed plan as the difference between the two is within the tolerance for that base scale? If it does, then we can normally accept that OS feature and plot to it because we can regard it as an accurate representation of the feature on the ground when displayed at the base scale. If the difference between the feature on the deed plan and the feature on the OS map is more than the tolerance for that base scale, we would not normally plot to the OS feature but would instead plot the full legal extent based on the deed plan.
If you feel that there are special circumstances relating to the application that you are mapping that mean that it may not be appropriate to rigidly adhere to the tolerance level for the base scale of the map in that area, please refer to a Plans HEO on the first instance to discuss. Further referral can be made to a Plans SCW in particularly complex scenarios.
The Plan Creator’s Check Information tool allows the plans officer to check the survey capture scale of each feature on the OS base map. The Accuracy figuren given allows us to quickly work out the capture scale of the features in that area of the map. This is very useful information as we often have to make mapping decisions based around the features on the OS map.
Examples
Example 1 - within tolerance |
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The east or rear boundary of this house plot measures 65 feet (19.8m) on the deed plan by reference to the scale bar. (Please note, the full plan contained a location plan so this is not a floating shape deed plan) | The occupational extent of the plot is fully defined on the OS map, and the rear boundary measures at 60 feet (18.3m). | The difference between the deed plan measurement and the OS basemap measurement of the rear boundary feature is therefore -1.5m. As the basemap scale in this area is 1:2,500, the maximum acceptable tolerance is up to 2m. So we can accept the OS depiction of the boundary as being the same as the boundary shown on the deed plan, and plot to that map detail when creating the cadastral unit. |
Please note, had the basemap scale in this area been 1:1,250 then the difference of 1.5m would have been greater than the maximum acceptable tolerance for that scale and it would not have been correct to accept the OS depiction as being the same as the plot shown on the deed plan. This could be an example of when occupational and legal extents differ, and it should be remembered that our role is to register the legal extent of properties in the Land Register. | ||
Example 2 - outwith tolerance |
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The deed for extent contains a full bounding description on the plot to be registered which can be summarised as -
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This full bounding description is acceptable for mapping from as it contains clear measurements and descriptions of the boundaries, as well as giving us a clear “starting point” to measure from - the corner of the stone wall at the junction of the southwest and northwest boundaries. | The occupational/fenced extent of the plot is partially defined on the OS map, with boundary features shown on the 3 of the 4 boundaries, but also with the gable of the neighbouring house shown on the 4th boundary. Comparing the deed description and the OS map, the boundaries are:
| The basemap scale in this area is 1:1,250, so the maximum acceptable tolerance is up to 1m. The two longer boundaries running roughly east-west are comfortably within that tolerance (0.3m and 0.2) and so the features shown on the OS map can be accepted as depicting the same features described in the deed. However the two shorter boundaries running roughly north-south have differences (1.5m and 1.6m) that are greater than the maximum acceptable tolerance for 1:1,250 scale so it would not be correct to accept the OS features as being the same as the legal boundaries defined in the deed. This appears to be an example of when occupational/fenced and legal extents differ, and it should be remembered that our role is to register the legal extent of properties in the Land Register. Since the deed lets us know that the SW and NW legal boundaries follow an existing stone wall, but that the SE boundary is the line of a planned fence (i.e. not built at the time of the deed), we can plot in the legal extent using the stone wall as our starting point. The end result is a cadastral unit that accurately reflects the legal description in the deed but does not extend as far south as the fence now shown on the OS map. | ||||||||||||||||||||
Please note, had the basemap scale in this area been 1:2,500 then the differences of 1.5m and 1.6m would have been within the maximum acceptable tolerance for that scale and it would have been correct to accept the OS depiction as being the same as the plot described in the deed. | ||||||||||||||||||||||
Example 3 - within tolerance at 1:10,000 |
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The deed plan included a scale bar, annotated measurements for the four boundaries of the plot, and a location plan, and therefore met the Keeper’s deed plan criteria. | The occupational extent of the plot is fully defined on the OS map. Comparing the deed plan and the OS map, the boundaries are:
| The basemap scale in this area is 1:10,000, so the maximum acceptable tolerance is up to 8m. This can feel like a big difference when you are used to mapping at 1:1,250 scale, however this tolerance level has been set because the accuracy of 1:10,000 scale OS maps is considerably different from the two larger scales. In this example, we are not concerned with the shorter northwest and southeast boundaries as these are very close in measurement to the depiction on the OS map. The two longer southwest and northeast boundaries, despite having differences of -3.5m and -4.5m between deed and OS map, are comfortably within the tolerance of 1:10,000 map areas (up to 8m) and so the features shown on the 1:10,000 scale OS map can be accepted as accurately depicting the same features shown on the deed plan, and we can plot to that map detail when creating the cadastral unit. | ||||||||||||||||||||
Please note, had the basemap scale in this area been 1:1,250 or 1:2,500 then the differences of 3.5m and 4.5m would have been greater than the maximum acceptable tolerances for those scales and it would not have been correct to accept the OS depiction as being the same as the plot shown on the deed plan. This could have been an example of when occupational/fenced and legal extents differed, and it should be remembered that our role is to register the legal extent of properties in the Land Register. | ||||||||||||||||||||||