Contours are indeed fascinating and in my professional experience the most useful feature on maps in both lowland and highland areas.
In the military navigation which I instruct, they area mainstay as they are the permanent feature, it is amazing that even after the explosion of the most powerful ordnance in the world, land-burst thermonuclear devices, the topography of the adjacent land which changes significantly is confined and surprisingly limited.
The way in which Ordnance Survey defines and depicts them is an inexact science, which has evolved as surveying techniques have advanced and reproduction techniques have improved.
The changes metrification brought in converting Imperial units to SI in itself presented further challenges to their cartographers, as the original Imperial measurements inevitably included mathematical error corrections which were compounded if the figures were simply converted into SI units.
As always, all credit to this excellent agency for producing the finest National cartography in the world!
Forgive this lengthy treatise, which is only of interest to contour nerds such as me The current OS Standards for contour interpretation and depiction are:
Contours 1:25 000 (1 to 25,000) aka Explorer maps:Contours are shown by orange lines and values in 50% monet magenta and 70% monet yellow.
The vertical interval (VI) of contours are either 5m or 10m. At 5m VI, contours at 25m, 50m, 75m etc, are accentuated; at 10m VI, contours at 50m, 100m, 150m etc, are accentuated.
Updating of contours is only in the case of sheets falling in a subsidence area or where old quarries and pits have been re-landscaped and later information is available.
The number of contours can be reduced where they are likely to block in when they are printed.
Make up any unwanted breaks in contour lines.
Digitally produced contours are available from the Land-form Profile.
When both 5m and 10m contour vertical intervals occur within the component 1:25 000 maps of reformatted sheets, the most suitable interval for the sheet is selected.
Holes In ContoursThe following actions explain holes in the contours:
• If it is clear that any part of the area within the hole is active, the description “Workings” is applied centrally in the hole on the Contour file (raster colour 7);
• If the area can be positively identified as no longer active and the area has not been relandscaped, the description “Disused Workings” is applied centrally in the hole on the Contour file (raster colour 7);
• If the area has been relandscaped and disused workings are no longer evident the hole is left unannotated. If contours are available they are re-instated.
Where detail on the source material, e.g. pit, quarry, corresponds to the hole in the contours the cartographer will ensure that contours do not encroach on the outer perimeter of the detail, pecks, cliff or slope symbol, which portray the feature. Small portions of contours in close proximity to the hole are omitted in cases where they could be misleading to the map user.
Detail which is shown within a hole in the contours or within the area of an active working are not deleted unless specifically cancelled on the LS document.
If there is evidence of either partial re-landscaping or reversion to normal use, and the contouring date is the same as, or later than, the survey date of the source material, then such contours are re-instated if available.
Contours In Coastal Slopes, Cliffs, Cuttings, EmbankmentsShow these contours where running parallel with and through such features. Contours crossing at an angle to the depicted slope or cliff are omitted. Cases of doubt are referred to the QC.
Contours crossing small bank features, e.g. tumuli, are shown straight across such features where the height change is not significant.
Contours Crossing Quarries, Pits, Sand DunesWhere contours are shown crossing quarries, pits or sand dunes they are omitted.
Contours Crossing Sand, Shingle, Saltings, MarshWhere contours are shown crossing areas of sand, sand and shingle, saltings or marsh they are retained.
Contours Crossing Roads, RailwaysContours are not shown across motorways or dual carriageways.
Contours are shown crossing roads other than motorways and dual carriageways unless there is more than a contour interval difference between the road level and the adjoining ground level.
Contours are not shown across railways.
Contours In Relation To Water FeaturesContours are not shown crossing double water features.
Contours not in sympathy with lakes, ponds or running water features are omitted or adjusted.
Contour ValuesContour values are shown; discretion may be exercised with regard to the positioning and density of values, but within the following rules:
• Sufficient values are applied, without overcrowding the map, to enable the user to find the height of any contour quickly and easily;
• Whenever possible values are applied in ladder formation continuing over hills and through valleys as necessary. Such formations may be curved, slanting or straight;
• The maximum distance between contour runs are 10cm. In areas where the terrain is rugged and irregular, contour runs will tend to be closer than 10cm;
• Values are positioned to read uphill; they are placed on straight parts of contours and clear of detail, names and vegetation;
• Each value is placed astride the contour line which is broken to accommodate the value and leave a clearance on each side of 0.5mm. In the case of small isolations the value is placed adjacent to the isolation in the horizontal position;
• All contours cut by the sheet edge, all depressions and those isolations which need identification are numbered except where space is limited or the existence of a levelled surface height makes numbering unnecessary;
• Adjust or omit values to clear grids, names and detail. Rock ornament however may be cleared to accommodate values; and
• Remove contour values where duplication occurs due to reformatting.
Contour Vertical Interval NoteWhere the slope angle is less than degrees contours appears are at 5 metres.
Contours 1:50 000 (1 to 50,000) aka LandRanger maps:
Metric contours at 10 metre vertical interval are shown on all Landranger maps. They are shown in screened orange. See “Screens” etc specification. Contours are derived from the following sources:
• Contours surveyed at 1:10 000 scale and at 10m vertical interval.
• By interpolation from contours surveyed at 1:10 560 scale which are at 25ft vertical interval.
(The following sheets in Northern Scotland still contain interpolated contours: 10, 11, 12, 15, 16, 17, 18, 19, 20, 22, 23, 24, 25, 26, 32, 33, 34, 35, 36, 39, 41 and 42).
• Field Revision.
• Development plans
Depressions, other than those indicated by zero height contours, are not shown.
In areas of close contouring the accentuated contours are given precedence, and some or all of the intermediate contours are omitted or broken depending on the space available.
The minimum distance between contours is not less than 0.4mm and the intermediate contours are normally omitted in the order lowest, highest, second lowest, second highest.
DepictionContours are shown as follows:
Those with a value of a multiple of 50 metres are line gauge 0.3mm
Other contours are line gauge 0.1mm
Zero height contours are line gauge 0.3mm
The minimum isolation size is 0.8mm.
The minimum clearance between contours and water is 0.2mm. Contours not in sympathy with lakes, ponds or running water features are omitted or adjusted.
Contours are shown in built-up areas.
Contours are not shown through areas of sand dunes and are broken where crossing motorways, cuttings and embankments, slope symbols, quarries, chalk pits, spoil heaps, refuse tips, dumps, landfill sites, opencast workings and double line water.
Contours are not shown through coastal slopes but are shown through coastal cliffs where running parallel with and through them. Contours crossing at an angle to the depicted cliff are omitted.
Contours are not broken for natural rock features.
Where contours are shown crossing areas of sand, sand and shingle, saltings or marsh they are retained.
Holes In ContoursThe following actions explain adding new contours and holes in the contours:
When any spoil heap, opencast working etc becomes inactive, the original contours are not reinstated unless verified by Change Intelligence. Where workings are complete and the land has been restored and landscaped and new contouring is available, it is supplied through Change Intelligence using OS data, contractors’ plans or field survey. Digitally produced contours may be available from the Land-form Profile.
In areas of active opencast workings the description “Opencast Workings” is placed central to the area. In opencast workings which are no longer active but there are no new contours available, the description “Former Opencast Workings” is used.
Where detail on the source material, e.g. pit, quarry, corresponds to the hole in the contours the operator will ensure that contours do not encroach on the outer perimeter of the detail, cliff or slope symbol, which portray the feature. Small portions of contours in close proximity to the hole are omitted in cases where they could be misleading to the map user.
Contour ValuesContour values are on the same layer as the contours so if new values are added the contour will need to be broken.
All values are in metres.
Sufficient values are shown to enable the map user to find the height of any contour quickly and easily but without obscuring map detail or generating clutter.
The values are placed where easily visible and so that the map user does not have to trace a contour a long distance in order to find its value. The use of spot heights in finding contour heights is taken into account. Values are not necessary on small isolations containing spot heights.
In closely contoured areas, it is sufficient to number only the accentuated contours but in open and flat areas all contours are numbered. Contour values are shown in ULR 6pt on the Contours File.
The following points provide a guide:
* Values are always placed to read uphill.
* Values are not placed on sharply curving parts of contours, or where the depiction of the relief feature is affected adversely.
* The values are placed in "runs" or "ladders" where possible. The runs are curved, slanting or straight and not closer together than 7.5 cm nor further apart than 15 cm.
* The values in the runs are not placed closer together than 6mm, centre-to-centre.
* Where possible the position of values and runs are made easily readable from the south edge of the sheet. However this is not always possible and some have to be placed “upside down”.
* Each value is placed astride the contour line, which is broken to accommodate the value and leave a clearance on each side of 0.5mm. In the case of small isolations the value is placed adjacent to the isolation in the horizontal position.
Clashes with all black, blue and red detail, names and symbols, including rights of way should be avoided. Rock ornament however may be cleared to accommodate values.
Lastly, I love the fun the cartographers have had in sometimes creating imaginary contours and even writing their names in lines where separation would give little meaning. I will upload the examples I have been given by some of the cartographers at OS - Anonymously of course