NEXTmap - Data Accuracy
A full description of the accuracy of Intermap's NEXTmap Britiain product range can be found in the product handbook / user guide, attached to the main product page.The following is CENTREMAPSlive brief interpretation of that information supplied purely as a starting point for customers before choosing whether to consult the handbook. Decisions on data purchase should not be made on these comments alone.
About Digital Terrain Model accuracy:
Intermap carefully monitor the quality of their NEXTmap data products, testing it in various situations against locally available reference data of higher levels of accuracy. It should be noted that the given levels of confidence in the accuracy of the data represent upper limits in the achievable accuracy and that the user should always be aware that local deviation from the general accuracy may occur.
It should particularly be noted that the following circumstances are outside of these tests which are confined to unobstructed, moderately sloped terrain:
- Continuous and dense buildings with narrow streets - the lowest point may not be ground (i.e. raised)
- Continuous and dense tree cover - the lowest point may not be ground (i.e. raised)
- Rapid changes in terrain - may be lost unless a 'break-line' has been applied. The manual application of this process limits the extent to which it is implemented although it is hope that automated methods may be introduced in the future.
- Sloped terrain - slopes greater than 10 degrees cause reduced accuracy with a slope of 20 to 30 degrees doubling the error.
Stated accuracies:
| Measures of Accuracy |
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| Specifications |
Nominal |
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| Product Type | RMSE | 95% | Mean | Standard Deviation |
| I | 0.7 | 1.5 | 0.5 | 0.5 |
| II | 1.0 | 2.0 | 0.7 | 0.7 |
We recommend users refer only to the 95 percent confidence values and bear in mid that given these values, it is implied that 5 percent of all tested (unobstructed and moderately sloped terrain) lies outside of these accuracies.
About CENTREMAPSlive derived contours:
The contours supplied via CENTREMAPSlive are generated automatically for each purchase and as such merely act as a basic interpretation of the NEXTmap Britain 5 metre Digital Terrain Model on which they are based.
The contours are placed at 1 metre height intervals not as a statement of accuracy but rather to ensure the inclusion of some height information even in generally flat areas or where the user has purchased a small area.
Suggested contour intervals as discussed by Intermap in their own contour provision are shown here and indicate to what degree users should accept our own 1 metre supply:
| Terrain Characteristics | Contour Interval |
| Slope: predominantly less than 10 degrees. Areas of moderate, open terrain. |
2 feet (approx 0.6 metres) and greater |
| Slope: greater than 10 degrees and less than 20 degrees. Dependent on amount of vegetation. |
5 feet (approx 1.5 metres) and greater |
| Slope: greater than 20 degrees Dependent on amount of vegetation. |
20 feet (approx 6 metres) and greater |
Whilst this data provides incredible detail and value for money, we would always recommend undergoing a full topographic land survey (offering up to millimetre accuracy) before any significant investment in a site's development. These services including building and underground service traces are available via a number of CENTREMAPS parent companies so please contact us for more information.
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