15.6.4 Depiction of Relief

The third dimension (height) is represented on OS maps through a combination of line work, symbols and annotated values, all related to the OS National Height Datum at Newlyn in Cornwall.

[ABC]	Heights are expressed in two common ways in all three scales:
	Bench Marks (BM's) - are marks taking the form of a brass plate or cut arrowhead located on permanent features, e.g. living rock or building face. The horizontal bar represents the actual heighted point and BM's are represented on the map by an arrow symbol defining plan position, with the height expressed in metres, to two decimal places alongside.
Note:	When using Bench Marks to establish ground height it should be remembered that the horizontal bar of the cut shape is normally 0.3 - 0.5 m above the ground. In some instances however, the BM can be up to 2.0m above / below normal ground surface level.
	Spot Heights are not ground marked points, but are included in the map to indicate the nature of the terrain. To facilitate ease of location spot heights are in the centre of roads / tracks and located at road junctions, fence / wall junctions and at sharp changes in ground height.On the map they appear as a cross symbol with a height expressed in metres to one decimal place; on 1:10,000 maps all decimal values are rounded up / down to the nearest whole metre value.

[C]

At 1:10,000 scale the general nature of the terrain is depicted by Contours, i.e. lines joining points of equal height above Ordnance Datum. These 'form lines' are captured photogrammetrically and are shown at 5.0 m intervals in lowland areas and 10.0 m intervals in more mountainous areas. Smaller 'infill' areas are surveyed on the ground by Spirit Levelling.

Remember, the closer the contours, the steeper the slope!

15.6.5 Currency and Maintenance of Large Scale Mapping

Maps are representations of the real world - like photographs they are 'snap shots' in time, recording the scene as the surveyor finds it on the day of the ground visit. When the sun slips behind a cloud and shadow envelops the landscape, or a car moves off along the road - the photographed scene changes irreversibly. The same is true in mapping. The day after the surveyor has visited a map site a wall can be knocked down (accidentally or as planned), a further house can be added to the developing estate, the river channel can overflow and migrate across the flood plain - and the map is rendered unrepresentative and 'out of date'. To maintain the currency of the national mapping archive, the OS must monitor change and continually revise their mapping accordingly. It is rather like being a painter on the Forth Rail Bridge - no sooner have you completed one revision of the town or countryside …. when it is time to return to the beginning and start all over again!

Change however by its very nature can be sporadic, regular cyclical map revisions are not enough. A revision may miss major new developments and the map could remain wholly unrepresentative of the true 'ground' situation for several years, at least until the next scheduled mapping visit. To overcome this problem the OS have developed an intelligence monitoring programme, which collates planning information from a wide variety of sources in conjunction with 'change evidence' gathered by surveyors on the ground.

A programme of detail capture has been developed to respond more readily to identified change, prioritising work loads by map scale and the amount, or nature of the detail outstanding.

The following section deals with both the maintenance and quality of the data, which constitutes the map detail, depicted on OS maps.

15.6.5.1. Units of Change

OS has historically quantified and recorded the amount of change taking place on any basic scale map by the number of house units of change. One house unit equates to the complete mapping of one new house and all its associated features, i.e. peripheral fencing, garage, drive, etc.
Other types of change are measured as parts, or multiples of the basic house unit.

A few examples are tabulated below:

15.6.5.2. Maintenance of OS Maps

In order to keep the maps as up to date as possible the OS have adopted two approaches to the revision and maintenance of our large-scale map database.

15.6.5.3. Continuous revision

Continuous revision is the term used to describe the process currently applied in urban, rural and moorland areas to capture major changes to the landscape such as large housing developments, commercial developments and major road or railway construction. They are captured within six months of building completion or demolition. These changes are captured using either ground or aerial survey methods, or a combination of both.

15.6.5.4. Cyclic revision

Cyclic revision is the term used to describe the process of 'sweeping' through rural areas over a five year period and moorland areas over a ten year period, to capture general revision information which is of most significance to customers. Major changes to the landscape in these areas that occur between revision cycles are captured by the continuous revision process described above. Cyclic revision is undertaken primarily using aerial survey methods.

15.6.5.5. Quality of Land-Line data

The term 'quality' can mean different things to different users, one generally-accepted definition however is 'that it is a measure of performance against a pre-determined specification'.
The quality of Land-Line can be considered in terms of the following criteria :-

15.6.5.6. Lineage

Great Britain was completely remapped between the years 1946 - 1983. These maps were produced by a number of different methods producing a range of accuracies within the overall published tolerances for each scale in question.
Land-Line was digitised from these published maps in a programme which commenced in 1971
and was completed in 1995. These digital maps have been constantly revised within a digital environment since their initial capture. The quality procedures used were formulated in collaboration with the National Joint Utilities Group, (NJUG).

15.6.5.7. Content and Completeness

This is a measure of the correspondence between the real world and the specified data content.
At initial digitising stage, all Land-Line tiles were rigorously checked to ensure that no features included on the source map were omitted with reference to the product specification.

Subsequent revision of the Land-Line tiles include local quality control procedures to ensure that all captured data conforms to the current specifications. Data 'cleaning and enhancement' processes have also been carried out to enhance the geometric fidelity of the data, e.g. closing 'closed' polygons, removing 'spikes' and redundant points in the data.

15.6.5.8. Currency / Revision Policy

All detail is not equal …. certainly not in the eyes of the OS! Real-world detail is sub-divided into four categories and each category is assigned a different priority with regard to its frequency of capture.

The following table shows each category of change, its revision category and frequency of change:-

this topic continues …