1、STDeITU-R RECMN P-1058-2-ENGL L999 = 4855232 053b95b 578 Rec. ITU-R P.1058-2 99 RECOMMENDATION ITU-R P.1058-2 DIGITAL TOPOGRAPHIC DATABASES FOR PROPAGATION STUDIES (Question TTZJ-R 20213) (1994-1997-1999) The ITU Radiocommunication Assembly, considering that the application of propagation prediction
2、 models requires topographical information; that 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) c) prediction; d) e) that dat
3、a exchange is required between different administrations; that it is desirable to establish a worldwide topographic database, recommends I I 1 2 the information contained in 0 5 of Annex 1; 3 4 include details of the type and height of the ground cover; 5 topographic database. that topographic datab
4、ase coordinate systems should be determined according to 5 2 of Annex 1; that the horizontal spacing of data values in a topographic database should be determined taking into account that topographic databases should unambiguously identify sea and lake surfaces, including their heights; that topogra
5、phic databases should contain 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 prediction
6、s need to contain information 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 MHz, especially in urban areas,
7、 information about the location, 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
8、 also demand more detailed 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 sam
9、ples. It must be noted that 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 im
10、plication with regard to 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 COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling
11、 Services. - STDmITU-R RECMN P.LO58-2-ENGL L999 = 4855212 053b957 404 D 100 Rec. ITU-R P.1058-2 database applications. This implies a requirement for unnecessary storage capacity. Under such circumstances, it does not seem appropriate at present to develop a set of ground cover categories which woul
12、d 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 I seem likely to be worth further investigation. I No universal storage format can be proposed for similar reasons to those given above. However, it is c
13、onsidered 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 the database may be modified as more information becomes available and, with suitable changes to the interface softwa
14、re, the propagation prediction routines are unaffected. I In order to effect a satisfactory exchange of a topographic database, for example between administrations or from a supplier to a customer, it is essential either that suitable interface software is supplied with the database or that full inf
15、ormation about the database contents and storage scheme are supplied. 2 Coordinate systems Topographic data can be referenced to any of several coordinate systems. These normally fall into one of two major Categories: - angular coordinates, normally latitude relative to the Equator and longitude rel
16、ative to a reference meridian, normally Greenwich; - a rectangular projection applied to a particular area of the Earths surface according to a defined mathematical projection. The principal characteristics of these two systems can be summarized as follows: - latitude-longitude coordinates provide g
17、lobal coverage without discontinuity, but with a non-linear relationship between coordinate values and ground distances. In particular the scale factor between longitude and ground- distance varies with latitude; - rectangular projections approximate to a linear and scale-invariant relationship betw
18、een coordinates and ground distances over a defined geographic area, but must be redefined for different areas to avoid significant distortion. Many national mapping agencies adopt a rectangular projection for paper maps, and for this reason the most detailed topographic data for a given area are of
19、ten indexed at regular intervals of the local projection. Many national mapping systems are based on the transverse Mercator projection. The universal transverse Mercator (UTM) system is a set of such projections based on uniform definitions for different longitudes, with northings referenced to the
20、 Equator. This provides a useful degree of standardization. In some cases there is a preference for projections optimized for accuracy at a specific latitude as well as longitude, in which case individual values for earth elipticity are usually chosen in order to minimize errors. There are also a nu
21、mber of non-transverse Mercator projections. The most suitable choice of coordinate system can depend on several factors, including: - where the highest accuracy is important there is an advantage in retaining the source coordinate system, since conversion to a different system will usually result i
22、n a loss of accuracy; - when extracting short path-profiles data indexed to a rectangular projection provides a useful simplification, since a straight line in the coordinate-space will approximate to a straight line on the ground. The discrepancy when compared with a true great-circle path will dep
23、end on the projection system and the path orientation and length. As a general guide, a straight line in a rectangular projection is typically sufficiently accurate for propagation studies up to about 100 km. However, actual discrepancies will vary according to the projection in use, and will tend t
24、o be greater for West-East path orientations and at higher latitudes. Users should evaluate worst-case errors when laying- out path profiles as a straight line in a rectangular projection; - latitude-longitude coordinates are valuable in providing continuous coverage over wide areas. When great-circ
25、le geometry is used to avoid excessive non-linearity, the use of latitude-longitude coordinates avoids the need for many coordinate conversions to a rectangular projection. COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling Services - - -_ - STDm
26、ITU-R RECMN P=LO58-2-ENCL 1999 4855212 053b958 340 - Rec. ITU-R P.1058-2 101 Parameter Applicability In view of the above factors it is not practicable to recommend a single coordinate system for all purposes. For intemational coordination the use of latitude-longitude is recommended in view of its
27、ability to cover the entire surface of the Earth without discontinuities. In cases where a rectangular projection is needed on the basis of practicality, the use of UTM coordinates is preferable on the grounds of uniformity. Latitude-longitude UTM Other Complete Earth Most of Earth Usually local The
28、 above discussion is summarized in Table 1. Scale-factor variation Boundaries TABLE 1 Coordinate systems Varies with latitude Good approximation to Usually good approximation to constant constant None According to longitude Varies 1 Usually good approximation to square Good approximation to I square
29、 Grid cell shape 1 Curvilinear trapezoid 3 Geodetic datum A geodetic datum is the set of reference values upon which a coordinate system must be based. The WGS 84 datum, which is based on the GRS 80 geoid, is recommended for international coordination. 4 Database compatibility When combining topogra
30、phic or mapping data from different sources care must be taken to ensure compatibility. In general misalignments will occur unless all data are based on the same geodetic datum and coordinate system. 5 Horizontal spacing in a macroscopic database The value of horizontal spacing between data-points w
31、hich should be used in a topographic database depends upon the use to which the data will be put. It is not practicable to recommend a particular value. In practice horizontal spacings in the approximate range 20 m to 1 km, or the equivalent in latitude-longitude, are typical. Various propagation-pr
32、ediction models not only have different requirements for horizontal resolution, but differing sensitivity to changes in horizontal resolution. It should not be assumed that increasing horizontal resolution with a given propagation method always improves prediction accuracy. 6 Accuracy of terrain hei
33、ght data The accuracy of propagation prediction models can be strongly affected by the accuracy of tevain height data in a topographic database. The accuracy of terrain heights is typically expressed as a root-mean-square (r.m.s.) error value. Horizontal resolution, vertical accuracy, and the propag
34、ation method in use, will all affect the calculated result. In general, the more detailed deterministic propagation methods require greater resolution and accuracy in topographic data, but details will vary in individual cases. An r.m.s. error of 15 m in terrain height data has been found acceptable
35、 for many purposes. COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling ServicesSTD=ITU-R RECMN Pa1058-2-ENGL 1999 W 4855212 053b959 287 102 Rec. ITU-R P.1058-2 7 General principles of data storage for terrain heights Most current topographic data
36、bases used for propagation prediction and radio planning use 2-dimensional arrays of data at equal intervals in the chosen coordinate system, referred to as “gridded data”. This has the advantage that horizontai coordinates need only be provided for reference points, with most data consisting of sel
37、f-indexing arrays of height values. For rectangular projections the horizontal data-spacing will typically be the same throughout a complete database. For latitude-longitude coordinates the longitude spacing is sometimes increased in steps as latitude increases in order to keep the longitude scale-f
38、actor approximately constant. Gridded data storage is recommended for topographic databases used for propagation studies on the basis that it is simple and in wide use. The following information is provided as general guidance on other approaches to storing topographic data which may be found useful
39、. There is increasing interest in using other storage strategies for topographic data both to reduce storage space and in some cases to provide a more efficient representation of terrain height. Standard methods can be used to compress any topographic data, although in general greater compression ra
40、tios are available using specialized methods, not ali of which are error-free. Examples of compression for gridded data are: - the discrete cosine transform (DCT); - various forms of Huffman coding, which can be error-free, and which is particularly efficient if the difference between actual heights
41、 and a prediction of height from neighbouring points is Huffman encoded for storage; - the use of variable point-spacing according to terrain irregularity, which can be stored efficiently in linked-list form using quadtree nodes. Where terrain height data are available at a sufficient horizontal res
42、olution for irregularly-located points, which normally implies a surveying system selecting features such as ridge and valley lines, the triangulated irregular network (TIN) has a number of advantages. The method is based on storing both the horizontal coordinates and height of each point. It is als
43、o necessary to define a triangulation linking ail points in order to represent terrain as contiguous triangular facets. The triangulation may be stored explicitly, or implicitly for reconstruction during data-retrieval. It should be noted that the advantage of TIN relies on points linked to terrain
44、features, which in general will be irregular. Two points arise: - traditional cartography does not always provide such points accurately surveyed to an adequate resolution; - a TIN system derived from gridded data should employ a system to identify the more topographically-significant points. It sho
45、uld also be noted that ambiguous triangulations can exist for regulariy-spaced points. 8 Representation of terrain height data Gndded terrain height values may represent different aspects of terrain height: a) the highest, lowest, median, or other characteristic height for a square area of terrain h
46、aving a side equal to the horizontal data spacing; b) the height at the single point represented without providing information on heights elsewhere. The choice of how heights are represented affects both how path profiles of terrain height should be extracted from a database, and how the height info
47、rmation will interact with a propagation prediction method. It is not practicable to provide a general recommendation in this area. Methods of profile extraction are discussed in 0 9. COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling ServicesRec
48、. ITU-R P.1058-2 103 9 Profile extraction When drawing a profile between two arbitrary locations, few or none of the data points in a grid-based database will coincide exactly with the profile. Various methods are available for extracting terrain height data in such cases. The following are recommen
49、ded according to circumstances: - when the height data are in some way representative of a square area of land, as described in 0 8 a), data should be placed in the profile for each square through which it passes. Each profile point may be placed on the normal from the profile line to the corresponding data point, although this will not in general produce equally-spaced profile points. If the propagation method requires equally-spaced points, it is acceptable to move profile points to accomplish this; - when the height data represent only the height at each exact p
