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Base map |
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Base map |
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The base map can be produced from a single source or a compilation from several sources. Possible sources for base map materials:
Official maps at large scales
Photogrammetry is an important part of modern topographical mapping. Basic survey carried out by government map agencies in nearly all countries is now done from air photographs using stereo plotting.
They show the man made features very accurately, but at the most common scales (1:25000 and 1:50000) a lot of detail is omitted and the contours, especially in the wooded areas, are not very accurate.
Air photographs
The main use of air photography is for photogrammetric plotting, though it can also supplement the information on other base maps, if used with care.
The information available from an air photograph depends on many factors: atmospheric conditions (haze and pollution), time of year, time of day, type of vegetation cover, type of camera, flying height, etc.
Negative size (format) - The standard negative size of air photographs is 23cm x 23cm (9" x 9"). Different sizes are sometimes found depending on the type of camera used, for example 18cm x 18cm (7" x 7"). Contact prints are produced on paper or film (diapositives) for mapping purposes. A panel along one edge of the photograph records camera information, although this may not appear on prints.
Focal length of camera - The distance from lens to film is accurately calibrated in aerial cameras, 152.23mm is common. This information is essential for anyone using stereo plotting equipment.
Camera angle - There are three basic camera angles:
Photo scale - The optimum photo scale for O-mapping is about 1:15000 (double scale of the base map). The scale is expressed as the ratio between the focal length of the camera lens and flying height above ground level (not sea level).
Thus for air photographs at a nominal scale of approximately 1:15000 the aircraft would fly at approximately 2300m above the ground. The scale of each part of the photograph depends on the ground level - the closer the camera, the larger the scale. Thus the scale varies continuously across the air photograph.
Camera axis - For mapping purposes the axis of the aerial cameral must be near vertical (± 3°). If tilts are greater than ± 3° the photographs will probably not be usable in most stereo plotters.
Oblique photography is also flown. The camera axis is deliberately tilted to give a wider panoramic view and may include the horizon. Such photographs are not used for mapping.
Stereo cover - To observe the ground in three dimensions, each point must appear on at least two photographs. To ensure this the flight paths are flown to give 15% lateral overlap while the camera is electronically timed to take photographs giving a 60% longitudinal overlap. These overlaps allow for slight mistiming, camera tilt, changes in relief and slight changes in flying altitude.
Distortions - When using aerial photographs it must be remembered that a photograph is a distorted picture of the ground with some detail considerably displaced from their true position. The displacements arise from two sources: differences in relief and camera tilt.
The effect of relief or tilt displacement varies continuously over the photograph. There are situations where:
displacement will be small with
Stereoscopes - To examine a pair of photographs stereoscopically a device for viewing them is required. The simplest is the lens stereoscope. It is compact and small enough to be folded and slipped into the pocket. It consists of two lenses and two pairs of
mirrors mounted on a stand. The separation of the lenses is adjustable to match the user’s eye base, the pairs of mirrors are used to seperate the lines of sight to enable a pair of photographs to be placed side by side. To use the stereoscope, the photographs should be placed on a well lit and flat surface with the overlapping parts side by side. The stereoscope is placed over the photographs and their separation and rotation is adjusted until the same area is viewed through each lens. When the position is correct the ground should appear sharply and comfortably in three dimensions. Anyone with normal binocular vision should be able to use a stereoscope satisfactorily.
Ortophotographs
In many parts of the world orthophotographs are available. These are photographs corrected for both relief and tilt and are at a specific scale. The orthophotograph is produced by re-exposing the original photograph in narrow strips in an adapted stereo plotter. It can be used as an accurate base map although only the planimetric detail is visible. An orthophotomap is the orthophotograph with contouring superimposed and some overprinting of the classification of roads and other detail to help interpretation.
Forestry maps
In some countries forestry maps are available, especially for state managed forests. The quality of these maps varies considerably and their value has to be checked in every case. Normally, there are no contours on these maps.
Planning maps
All over the world a lot of maps at large scales (1:1000 - 1:5000) are made for planning and building. They often include contour lines and exact spot heights and bench marks. In areas near to villages or towns they may save a lot of work.
Digital map data
Many official map agencies are making their mapping available in digital form. There are two forms of digital data: raster image data and vector data.
Raster image data - is produced from scanning existing printed maps, typical resolution after scanning is 300 or 400 dpi. It should be remembered that the quality of the raster map image is reduced if it’s scale is modified. Large scale maps, 1:10000 or above are generally in raster image data format. Vector data - is produced by digitising an existing map and/or including data from current surveys such as digital photogrammetry and field surveys using theodolites with data capture devices. Features are stored as lines defined by a series of discrete X-Y coordinate points with possibly additional attributes such as height, feature code, whether the line is a closed polygon etc. This data is independent of scale. Vector data is generally available for large scale maps (planning maps) and can often be purchased in a variety of formats.
Photogrammetric Plots
Photogrammetric plots are produced from aerial photographs, a pair of adjacent photograph are placed in a stereo plotter and adjusted until a 3-D stereo model is visible. All features that are visible together with the topography of the terrain are then plotted.
Stereo plotting instruments - A stereo plotter is a high precision instrument in which a theoretical 3D model of the ground is formed from two overlapping photographs. The model is adjusted to the required scale and levelled so accurate height readings can be taken. The operator sees a pair of marks appearing to float in the field of vision, these marks can be moved over the model in any direction, horizontally and vertically. There is one floating mark visible in each eye piece. When looking at the model through both eye pieces together the floating marks can be moved together or seperated by adjusting the height counter. When the two marks appear as one, the mark is at the surface (ground, top of tree canopy, etc.) of the model and the position of the point can be plotted.
To plot linear detail the operator moves along the feature, raising or lowering the mark continuously to keep in contact with the ground. To plot contours the operator sets the height counter to a given value and moves around the model keeping the pair of floating marks superimposed (ie. at the surface). When viewing dense forest plantation it is impossible to view the ground and so the operator adjusts the height counter to compensate for the height of trees. Checks should be made each time the ground becomes visible (ie. crossing tracks and rides etc.). It should be remembered that trees do not all grow to the same height and so contours plotted from the tree canopy will be very approximate.
With analogue stereo plotters, the carriage with which the operator controls the floating mark is attached to a pantograph which draws the plotted detail on a drawing table to one side of the plotter. Nowadays, these plotters can be fitted with encoders so that data capture to a computer is possible, with such a plotter (or the more modern analytical stereo plotter), the base map can be supplied either as a hard copy - the traditional pen plot with coloured inks - or as a digital file, or both.
Orientation of the model - Some form of ground control is necessary. To scale the model at least three known positions are plotted on to a sheet of film. The model is enlarged or reduced until it fits the points. To level it there must be a minimum of four known heights in the area covered by the model. The model is tipped and tilted until its height readings correspond to the known heights. The source of information for scaling and levelling is usually the official topographic map of the area with exact and accurate spot heights.
When to use photogrammetric plots - In O-mapping the use of photogrammetry is determined by:
It is of course possible to produce a good O-map using only government agency maps, but with a good photogrammetric plot as a base it can be done in less time and more accurately. Photogrammetry though, has its limitations and on occasions there may
be circumstances when stereo plotting will not be good enough to produce a satisfactory base map (eg. in steep forests with dense coniferous plantations). The operator is only able to reproduce the details visible from above. Therefore the quality of the aerial photographs is critical. On occasions older photographs may be better than more recent ones - this will be when more of the terrain is visible: a less mature forest with smaller trees or early spring photographs with no leaf cover.Who should produce a photogrammetric plot - Most commercial photogrammetric plotting is done of open land and the resulting plot has a very high degree of accuracy and completeness. Photogrammetry for orienteering requires a specialist photogrammetrist who is familiar with the requirements of an orienteering base map and is used to plotting areas covered with trees.
The photogrammetric base plot - A photogrammetric plot is only a base map and it has to be changed into an orienteering map by many hours of fieldwork. All the features have to be checked and corrected where necessary. It also needs quite a lot of time to get familiar with the contents of a plot.
Cost of photogrammetry - The price depends on the quality and scale of the photographs available and the complexity of terrain.
Typical costs £75 - £150 per km2 (1997).
Previous Orienteering Maps
A previous orienteering map is an invaluable source of information. It will show the majority of features that will need to be included on the new map. Knowing approximately were features are will reduce the amount of time needed to field work the area. Previous course setters should be consulted to identify those areas that are well mapped and those that are badly mapped!
If the previous orienteering map was good there should be little need to use other base materials.
If a photgrammetric plot of the area had prevously been used then there is probably little point in commissioning a new plot.
General
Orienteering uses a great variety of areas, from dense conifer plantation to open moorland: the content of the base map will vary accordingly.
The choice of the base map depends on the many factors described previously. In many cases, a combination of different base materials can give the best solution, and it is worth spending some time on this aspect.
Orienteering map firms
List of firms and people specialising in photogrammetry for orienteering maps.
Further reading
Photogrammetry in Orienteering by Robin Harvey & Flemming Nørgaard, published by IOF, 1993
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