profile guidance

Land-Form PROFILE® guidance notes

Introduction

These notes provide guidance on the appearance of maps created from Land-Form PROFILE Contours and guidance on the Contour and Digital Terrain Model (DTM) versions of the Land-Form PROFILE data. Ordnance Survey produce a User Guide for these products which can be downloaded from here. The User Guide also includes details on the use of NTF and DXF to supply the data.

Land-Form PROFILE Contours have been created from a 1:10,000 scale Ordnance Survey Mapping. In turn, the Land-Form PROFILE Digital Terrain Model (DTM) has been mathematically generated from the Contours dataset. They are each very different ways of presenting height information. These guidance notes are therefore structured into two separate sections, one for Contours, and one for DTM.

All heights in Land-Form PROFILE are given in metres above mean sea level (or more precisely, above Ordnance Survey's reference datum, which was mean sea level at Newlyn in Cornwall measured from 1 May 1915 to 30 April 1921).

The Land-Form PROFILE products are updated annually.

Land-Form PROFILE® Contours

General

Land-Form PROFILE Contours contain line features and point features.

The line features depict the following:

Standard Contours: At 5 metre vertical intervals, except in mountain and moorland areas where the vertical interval is 10 metres
Index Contours: Where the vertical interval of standard contours is 5 metres, index contours are at multiples of 25 metres. Where the vertical interval is 10 metres, index contours are at multiples of 50 metres.
Mean High Water Tide Line, derived from O.S. Land-line source data (surveyed at 1:1250 scale in urban areas; 1:2500 in rural areas; or 1:10,000 in moorland and mountain areas)
Mean Low Water Tide Line, derived from O.S. Land-line source data (surveyed at 1:1250 scale in urban areas; 1:2500 in rural areas; or 1:10,000 in moorland and mountain areas)

The point features depict the following:

Spot Heights (captured by ground survey), derived from O.S. Land-line source data (surveyed at 1:1250 scale in urban areas; 1:2500 in rural areas; or 1:10,000 in moorland and mountain areas)
Air Heights (captured by photogrammetric methods [stereo interpretation of aerial photography] at the same time as capturing the contours).

Instances of each feature type carry a numeric attribute giving a height value to a resolution of 1 cm. In the case of Coastline this height is 0m. Some Form Lines in low-lying terrain (for instance in Norfolk) have negative values.

Land-Form PROFILE Contours contain no cartographic text (i.e. text designed for display on screen). The height attribute associated with each feature can be used for display purposes.

Accuracy

Digital contour accuracy values are typically better than half the contour interval, that is ±2.5 metres for areas with 5 metre vertical intervals and ±5 metres for areas with 10 metre vertical intervals.

Spot heights are recorded to the nearest 0.1 metre.

Air heights and some supplementary spot heights are recorded to the nearest 1.0 metre.

Feature Codes

The Land-Form PROFILE Contour data contains six feature types. Each of these is given a Feature Code which in NTF data is a text string made up of four digits. Features codes, or layer names, in Land-Form PROFILE Contour DXF data, are prefixed by the characters "G802". For more information on DXF data download the user guide. The complete list of Feature Codes for Contour data and a description of what they refer to is given here:

Feature Code	Description	Type	Attributes	Notes
1372	Air Height	Point	HT	Height measured at a given point, using photogrammetric techniques. Air Heights are usually located at the top of hills, where Spot Heights do not exist.
0027	Spot Heights	Point	HT	Height measured at a given point, from ground survey. Spot Heights are usually located at the top of hills.
1361	Index Contours	Line	HT	when vertical interval of standard contours is 5 metres, index contours are at multiples of 25 metres. When vertical interval is 10 metres, index contours are at multiples of 50 metres.
0075	Standard Contours	Line	HT	At 5 metre vertical intervals, except in mountain and moorland areas where the vertical interval is 10 metres
0071	High Water Level Tide Line	Line	HT	Mean High Water along coast, including tidal estuaries.
0072	Low Water Level Tide Line	Line	HT	Mean Low Water along coast, including tidal estuaries.
0059	Water			No features with this feature code appear in the PROFILE dataset

The attribute code used above is detailed in the table below.

Code	Description	Notes
HT	Height	Height of feature in metres.

Air and spot heights (Feature Codes 1372 [air heights] and 0027 [spot heights])

Spot Heights

Heighted points captured by ground methods. They are normally positioned along the centre of roads.

Air Heights

Heighted points captured by photogrammetric methods at the same time as the contours. They are normally shown in the following positions:

Hill tops
At the highest point
Saddles
On important features
Valley junctions
Generally at the junction of main rivers and streams
River crossings
At natural ground level
Roads
At marked changes of gradient, or at about 500 metre intervals along roads with no marked change of gradient
Road junctions
Confined to main road junctions in town areas. Minor road junctions are included in rural areas
Artificial features
In confined places where slopes are depicted by symbols instead of contours a single central spot height is captured. On extended features, spot heights are captures at intervals of 750 metres
Low-lying areas
In large areas between the lowest contour and mean high water, supplementary spot heights are plotted at a frequency of about 750 metres
Flat areas
In extensive areas devoid of contours, spot heights are positioned on any eminence or significant detail.

Contours (Feature Codes 0075 [standard Contours] and 1361 [Index Contours])

Contours are depicted as two dimensional lines, with the HT attribute giving the contour's height in metres.

The Standard Contours are at a vertical interval of 5 metres (With an Index Contour every 25 metres), except in "mountain" or "moorland" areas where the vertical interval is 10 metres (with an Index Contour every 50 metres. The same vertical interval is applied to an entire Ordnance Survey map tile. Therefore a tile will include 10 metre intervals, or 5 metre intervals, but will not include both. Where a map tile containing 5 metre vertical intervals meets a map tile containing 10 metre vertical intervals, alternate 5 metre contours will end abruptly at the tile boundary, as shown in the following image of Land-Form PROFILE tiles SH90SE and SJ00SW:

Tile SH90SE, on the left, has contours with 10 metre vertical intervals because all or part of the geographical area covered by the tile includes "mountain" or "moorland" areas. Tile SJ00SW, on the right, contains no "mountain" or "moorland" areas and so has contours with 5 metre vertical intervals.

This lack of edge matching of contours across tile boundaries is not an error in the data but the result of a data capture and formatting decision made by the Ordnance Survey.

Contours in Land-Form PROFILE are also discontinuous in some locations for the following reasons:

in areas of steep slope some contours will be omitted. This is usually because the contours were missing from the graphic product (from which PROFILE was digitised) for cartographic purposes
major road and railway cuttings and embankments
Where coincident with man-made features, contours may be broken. Again, this is usually due to the 1:10,000 scale mapping used to create the PROFILE product not showing contours where they coincided with man-made slopes
Contours are not shown in active quarries, gravel pits spoil heaps or open cast mines.

Tide Lines (Feature Codes 0071 [high water level] and 0072 [low water level]

Tide lines include both high water marks and low water marks. In England and Wales these represent high and low marks of a mean tide, which is an average tide halfway between spring and neap tides. In Scotland the high and low water marks are of average spring tides. A Tidal Line will cross tidal rivers at the point to which mean tides (England and Wales) or spring tides (Scotland) flow at high water (known as the Normal Tidal Limit).

Water (Feature Code 0059)

Although a "Water" feature code [0059] appears in the NTF file header information for Land-Form PROFILE Contour data, the feature does not appear anywhere within the dataset. Thus, when viewing the PROFILE Contours in a GIS, a "Water" feature layer may be present, though there will be no features within it to display.

Land-Form PROFILE® Digital Terrain Model (DTM)

General

Land-Form PROFILE Digital Terrain Model (DTM) contains height information only. It does not contain a representation of the Spot Heights, High and Low Water Marks or other features that are represented in Land-Form PROFILE Contours.

Heights are given at the intersections of a horizontal 10 metre grid. These values have been mathematically interpolated from the same set of features that are represented in Land-Form PROFILE Contours. The results achieved will depend on the density of height data contained in the contour file.

In some flat areas where there is little height information, contours and spot heights may be a great distance apart. This can cause irregularities in the DTM which appear as slight terracing of the terrain.

Accuracy

The height accuracy of any point in the DTM is equal to or better than half the contour interval. that is ±2.5 metres for areas with 5 metre vertical intervals and ±5 metres for areas with 10 metre vertical intervals.