1、W 4855212 052338b 14T RW. ITU-R PN.1058 RECOMMENDATION ITU-R PN. 1058 67 DIGITAL TOPOGRAPHIC DATABASES FOR PROPAGATION STUDIES (Question ITU-R 202/3) (1994) The ITU Radiocommunicaion Assembly, considering that the application of propagation prediction models requires topographical information; that
2、future propagation prediction models will be able to make use of more detailed topographic information; the need to provide practical engineering advice on the preparation of digital topographic maps for propagation a) b) cl prediction; d) e) that data exchange is required between different administ
3、rations; that it is desirable to establish a worldwide topographic database, recommends 1. spherical coordinate grid system; 2. 0 2 and 3 of Annex 1; 3. 4. include details of the type and height of the ground cover; 5. topographic database. that topographic databases should use either the Universal
4、Transverse Mercator or a latitude-longitude that the horizontal spacing of data values in a topographic database should be determined in accordance with that topographic databases should unambiguously identify sea and lake surfaces, including their heights; that topographic databases should contain
5、information about ground cover, either man-made or natural, and that the additional information contained in Annex 1 should be taken into account when setting up a ANNEX 1 1. Introduction Digital topographic databases established for the purpose of propagation predictions need to contain information
6、 which is related to the type of prediction being undertaken. For frequencies above about 30 MHz, information about the terrain height and ground cover is currently needed. For detailed propagation predictions for frequencies above about 1 O00 MHz, especially in urban areas, information about the lo
7、cation, size and orientation of individual buildings is currently needed in addition to terrain height information. It is to be expected that increasingly sophisticated prediction models will be developed which will permit more detailed propagation predictions but which will also demand more detaile
8、d information and, potentially, a reduced horizontal spacing for the data samples. The purpose of this Annex is to provide guidance on the type of information which should be contained within topographic databases and on suitable values of horizontal spacing for the data samples. It must be noted th
9、at a very wide range of uses for topographic databases can be foreseen and also that a very wide range of ground cover information can be identified. In any individual geographic region, it is unlikely that all types of ground cover will be found and this has an important implication with regard to
10、the data storage. While a universal set of ground cover information could be developed, many of the categories would be irrelevant in the majority of specific topographic database applications. This implies a requirement for unnecessary storage capacity. Under such I 4855232 0523387 08b 68 Rec. ITU-
11、R PN.1058 circumstances, it does not seem appropriate at present to develop a set of ground cover categories which would be used in the same way in all applications. Guidance can, however, be given on the categories which have been found appropriate and those which seem likely to be worth further in
12、vestigation. No universal storage format can be proposed for similar reasons to those given above. However, it is considered to be desirable that propagation prediction computer routines should access the database by means of suitable interface software. In this way, the contents and structure of th
13、e database may be modified as more information becomes available and, with suitable changes to the interface software, the propagation prediction routines are unaffected. In order to effect a satisfactory exchange of a topographic database, for example between administrations or from a supplier to a
14、 customer, it is essential either that suitable interface software is supplied with the database or that full information about the database contents and storage scheme are supplied. 2. Horizontal spacing values in a macroscopic topographic database It is generally most convenient if terrain height
15、values are stored for locations which form a regularly spaced grid over the whole of the geographic area under consideration. Standard compression techniques may be used to reduce the data storage for areas of uniform height, for example, seas or lakes, but it requires extra processing to extract th
16、e data. In most cases this is likely to be fairly small compared with the time taken to access data on a mass-storage device (usually optical or magnetic disk). Considerable savings may be made by relating the horizontal spacing of the values stored to the location-to- location variation of the elem
17、ent being stored. For example, if there is a fairly uniform mean terrain slope with little variation from the mean value over a large area, it may be worthwhile to consider a special storage scheme which minimizes the space required and the access time needed. There is relatively little experience o
18、n which to base any proposals in this respect. Most topographic databases use a regularly-spaced grid for the whole area under consideration and rely on standard data compression techniques to deal with cases where the data values are constant. In practice, horizontal spacings in the range 20 to 1 O
19、00 m, or the equivalent in arc-seconds, are in widespread use. A value of 50 m has been found to be suitable where the geographic area contains some mountainous regions (mean terrain slope in excess of 40%). A value of 500 m has been found to be suitable for flat terrain (ground slope of 4% or less)
20、. However, to allow for future development of propagation prediction methods, it is advisable to choose smaller horizontal spacing values rather than larger ones. In principle, there is no reason why the horizontal spacings for ground cover and terrain height values should be the same; however, in p
21、ractice this is found to be more convenient. 3. Urban area topographic databases Special considerations apply in the case of topographic propagation predictions in urban areas, especially at frequencies above about lo00 MHz where reflections from building surfaces need to be taken into account. In s
22、uch cases, very detailed information is needed, including the height and shape of the buildings and possibly, the location and width of the streets, although the latter information may, in some cases, be derived as the area not covered by buildings. Two different approaches have been found useful in
23、 such cases. The first approach is an extension of the general concept outlined in 5 2 using a horizontal spacing of 5 to 10 m and storing the building height information as part of the ground cover data (see Note 1). In this case, the mean orientation of the buildings may only be derived approximat
24、ely. In the second approach, a vector database is set up which holds the coordinates, in three dimensional form, of the vertices which define the shape of each building (see also 5 5). In both cases, the heights stored may be absolute values or values relative to the local mean ground height. There
25、is currently insufficient evidence to indicate which of these two approaches provides the better basis for propagation calculations. Note I - Irregularly shaped buildings may be stored as a combination of sub-buildings with different heights. Rec. ITU-R PN.1058 69 4. Macroscopic ground cover informa
26、tion As noted earlier, the range of possible ground cover categories is very large and the full range is unlikely to be relevant to any individual geographic area. Table 1 gives a set of general categories which are in use by various organizations. However, in many cases, it has been found desirable
27、 to subdivide some of the categories to provide a better description of the ground cover involved. TABLE 1 Categories and parameters to be listed in macroscopic ground cover database Ground cover Dense urban (City centre location) Urban (Areas with commercial developments offices, shops) Industrial
28、(Areas with factories and warehouses) Dense suburban (Areas with terraced houses greater than 3 storeys or other high-rise dwellings) Suburban (Areas with detached and semi-detached dwellings, housing estates) Village centre (Typically locations with a green area and low density buildings) Deciduous
29、 trees Coniferous trees Mixed tree types Orchard Sparse (Featureless region such as fields, parkland, sand dunes, etc.) Marshland Mud flats Sea water Parameters 1) Mean building height 2) Maximurn buildin height 3) Building density(? 1) Mean building height 2) Maximum buildin height 3) Building dens
30、ity( 4 1) Mean building height 2) Maximum buildin height 3) Building density(? 1) Mean building height 2) Maximum buildin height 3) Building density(4 1) Mean building height 2) Maximum buildin height 3) Building density( 4 1) Mean building height 2) Maximum buildin height 3) Building density“ 4 1)
31、Mean tree height 2) Maximum tree height 3) Tree density(2) 1) Mean tree height 2) Maximum tree height 3) Tree density(2) 1) Mean tree height 2) Maximum tree height 3) Tree density(2) 1) Mean tree height 2) Maximum tree height 3) Tree density(2) Fresh water (*) Tree density is defined as number of tr
32、ees per 10000 m2. 4855232 0523840 875 Ground cover category Row of buildings (A well defined row of buildings in isolation, typically a row of terraced houses along a road) 70 Rec. ITU-R PN.1058 Parameters 1) Mean building height 2) Coordinates at end points of row 5. Special purpose ground cover in
33、formation The considerations in 0 3 may be extended to any special situations where detailed propagation predictions are required. Table 2 provides some examples of categories of ground cover together with a possible mechanism for recording their characteristics; this is basically an extension of th
34、e first approach described in 0 3. Alternatively, the vector method of storing the coordinates of the extremities of the objects may be used. Isolated building (Isolated building within a square) Line of trees (Typically a tree-lined road) TABLE 2 Additional categories and parameters for database of
35、 special structures 1) Building height 2) Coordinates of building centre 3) 1) Mean tree height 2) Area covered by the building Coordinates at ends of tree line Towers (Electricity pylons, wind turbines, etc.) 1) Height of feature 2) Coordinates at centre of feature I 6. Population data For inany purposes it is necessary to establish the population coverage for a radiocommunication service. This can be done very conveniently if a population data bank is established and it has been found useful to have the same horizontal spacing for population as for ground cover and terrain heights.
