Background of the Invention A digital terrain model requires to use a description of an array of points characterizing the respective area, whereby, each point is characterized by its X-, Y-and Z-coordinates, as its input data, i. e. a digital model of its relief and other data specifying its terrain nature in a greater detail, i. e. a digital model of territory. Such data may be a relatively large database.

Quite a number of firms active in development of CAD systems oriented to processing of geographic data offer techniques for making digital terrain models. There are substantial differences with respect to efficiency between individual programs solving the problems of modeling a digital terrain model. Input information processing rate is an important factor of a digital terrain model. Other factors being especially the work with a model (a possibility to identify objects displayed in the digital terrain models, determination of the distance between objects, finding out whether two object are mutually visible, bearing determination of any object from the viewer position, displaying only the determined objects, etc.). Frequently, a high efficiency puts high demands on the hardware used what is obviously reflected in the acquisition costs of a computing system.

But how to solve a situation when for a certain circle of users a user-specific application software is developed and a terrain visualization is required but only as a supplementary application which should provide only certain functions. Implementation of a ready-made product into a so developed software product can be a rather complicated matter, it will be burdened by costs of acquisition of the ready-made visualization product and the capabilities of the respective visualization product will be utilized only to a small part, whereby, some other functions may be required by a particular user subsequently and the already purchased product may not be able to realize them.

Summary of the Invention The above mentioned drawbacks are removed by a method of making interactive digital terrain model according to this invention, which model is processed by a computing equipment and made from a digital relief model, which relief model is transferred into a regular square grid of points determined by their X-, Y-and Z-coordinates and from a digital model of territory, which model of territory comprises information about individual geographic layers of this area. The principle consists in that the altitude of the respective point of the digital relief model is completed by an attribute A" from the digital model of territory which attribute"A"provides information about the respective priority geographic layer of the respective area in the respective point and, thereby, a minimized data file for displaying of a digital terrain model is formed. An output of this method is a modified digital terrain relief where the array of points describing the given area is now in the form of the elements of data X, Y, Z, A, where A is the attribute of the respective priority geographic layer and the X, Y, Z are the original coordinates from the digital relief model. Thereby, the database is reduced from the original digital relief model and the digital model of territory to a modified digital relief model.

The digital terrain model is drawn in cross sections beginning with the one which is at the greatest distance to the actual viewer position. Pieces of data about altitude are transformed together with the coordinates X, Y into the coordinates of the displaying surface and they form vertices of a polygon in the displayed cross section. The polygon is filled with a color during its displaying, whereby the color shade becomes darker with longer distance and lighter with shorter distance of the polygon, whereby, plasticity of the image is obtained.

On the basis of the attribute, A" from a data file for visualization of a digital terrain model an icon of a pre-defined shape and of an exactly defined color characterizing the object type is assigned to individual objects of the digital terrain model.

Coordinates of an object with an assigned icon are transferred into a color form and they are placed into an exactly defined icon spot for a reverse object identification in the original digital model of territory.

Brief Description of the Drawings To facilitate understanding of this invention Figure 1 is attached showing as an example an object icon which object is a church, whereby, the icon color pixels, the icon color field vertex pixels and the position information carrying pixels are shown.

Detailed Description of the Invention The invention was embodied as a digital relief model of a regular square grid 100 by 100 m.

It is a digital relief model where an altitude value is assigned to each point every 100 meters. The whole area of interest is divided into partial 10 by 10 km files. The integer data are in binary form, i. e. each altitude piece of data is a 2 byte long. The maximum altitude value in the file does not exceed 2047 m, data beyond the space of interest are set to zero, no piece of data is a negative value. Every file is determined by the respective coordinate of the south-western corner of the square 10 by 10 km and the X-, Y-coordinates of the respective altitude value are specified by their position in the file. Size of the digital relief model database is about 60 MB.

The other input database is the digital model of territory in the scale 1: 200,000. The database of the digital model of territory is articulated into individual geographic layers. The individual geographic layers are articulated into partial files so as the objects belong to the respective map sheet in scale 1: 100, 000.

Individual layers of the digital model of territory are so interrelated with the digital relief model database files so that attributes"A"are assigned to the respective altitudes according to predetermined priorities. The assigning of attributes can be realized by two possible methods. One method consists in that an altitude piece of data is enlarged by adding additional 2 bytes and the other one consists in that the so called"free"bites of the 16 bites necessary to specify an altitude are used. Thereby, an array of attributes is obtained which array is used to record the code of the layer which corresponds to the given altitude.

The former method, when the altitude piece of data in the digital relief model is enlarged by additional 2 bytes makes possible to assign information about up to 65535 different layers or their modifications or combinations to the altitude. But at the same time, it doubles the size of the original file specifying the given area.

The latter method, when the so called"free"bites are used, is based on the fact that in a given area 11 bits are enough to specify any terrain altitude ([211]-1 = 2047). The remaining 5 bites (always 0, thus free") make possible to assign 31 different layers or their combinations to a given altitude.

Thereby, there no increase in the database volume of the digital relief model takes place.

A result of such filling of the attribute array is a modified ordered set of points representing altitudes of the digital relief model including attributes of layers from the digital model of territory. The so modified files of the digital relief model, in their minimized form, are ready to be used in displaying the digital terrain model.

In the next part a digital terrain model is drawn. In order to ensure that the color we want to display corresponds to the color that is really displayed it is necessary to calibrate the colors. Color calibration consists in that tables of the brightness components R, G, B are formed. A method of preparing a table for the red brightness component is shown in Example 1.

Example 1 Preparing of a red brightness component table: Step 1: Output brightness component Rout =-1; Input brightness component Rin = 0; Pointer in table set to the beginning of the table ; Step 2: Previous brightness component Rold = Rout ; Step 3 : If Rin is higher then 255 go to Step 5, else display color pixel by means of macro RGB (Rin, 0,0); Step 4: Read pixel and evaluate red brightness Rout component ; If Rout is different from Rold, input Rout to the brightness component table, increase pointer in table by 1, Increase Rin by 1 and go to Step 2; otherwise increase Rin by 1 and go to Step 3; Step 5: End By preparing the individual brightness component tables independence of the graphic adapter and the operation system used is obtained. On the basis of the current position and the direction chosen a digital terrain model of the required area is drawn, particularly an area of 12 by 42 km size, i. e. more then 50,000 ha. Drawing is carried out so that cross sectional cuts are formed beginning with those that are most distant to the current position proceeding towards the actual position. It is so to solve the terrain visibility problem. During the process of making a cross sectional cut of terrain, data on altitudes modified by attributes"A"of the geographic layers are read from the input file. The read piece of data is separated into two parts, the altitude value and the geographic layer code.

The information on the object position in the XY plane of the geographic system is specified by the X-and Y-coordinates. In displaying an object on a computer screen, the XY plane coordinates are transformed into the UV plain into U-and V-coordinates. These numeric values of coordinates are transferred to colors with brightness components Red, Green, Blue. Thus, the numeric value of the X-coordinate in the XY plane is transformed into the UV plane into a numeric value of the U-coordinate and this numeric value of the U-coordinate is transformed to a color having the brightness components RedU, GreenU, BlueU. Similarly, the numeric value of the Y-coordinate is transformed into a color having brightness components RedV, GreenV, BlueV. Therefore, the numeric values of the X-and Y-coordinates are transformed into two colors and for displaying them on a computer screen only 2 pixels are sufficient. Therefore, it is true that: [RGB (RedU, GreenU, BlueU), RGB (RedV, GreenV, Blues) if (U, V) =g (X, Y); X, Y Coordinates of the X, Y plane of the geographic system of coordinates; U, V Coordinates of the U, V displaying plane ; RedU, GreenU, BlueU Brightness components of a color specifying the numeric value of the component U, resp. X; RedV, GreenV, BlueV Brightness components of a color specifying the numeric value of the component V, resp. Y; RGB Macro making a color composition of individual brightness components.

On presumption that transformation of the X-, Y-coordinates of the geographical system of coordinates into the U-, V-coordinates of a displaying surface was carried out and that the tables of brightness components were filled in, the numeric values of the object coordinates are transformed into a color form and they are placed into an exactly defined place of the icon for reversal identification of an object in the original digital terrain model. This transformation is carried out by means of indexes of the brightness component tables.

Let NR be the number of elements in the table for red, NG that for green and NB that for blue brightness components. Indexes pointing into the tables to their individual elements let be IRU, resp. IRV, IGU resp IGV, IBU resp. IBV.

Then for the U-coordinate the indexes IRU, IGU and IBU are determined by means of the following relations (where/is the operator for division, * is that for multiplication,-is that for subtraction): I RU = integer value (U/(NG * N B)) IGU = integer value (U-(U-IRU*NG*NB)/NG) ; IBU integer value (U-IRU*NG*NB-IBU*NG) ; Similarly for the V-coordinate: <BR> <BR> IRV= integervalue (V/(NG*NB))<BR> IGVinteger value (V- (V-IRV*NG*NB)/NG) ; IBV integer value (V-IRV*NG*NB-IBV*NG) ; The above specified relations are valid on assumption that the U-coordinate resp. the V-coordinate are not greater than the product of NR*NG*NB.

Consequently, to express e. g. the numerical value 29546 the graphic adapter has to be set in the mode for displaying at least 32*32*32 = 32K colors.

After determination of indexes of the brightness component tables, the numerical values of the X-components, resp. those of the U-and Y-resp. the V-components, can be expressed by colors by means of the RGB macro, where Rtab, Gtab and Btab are tables of the brightness components: X-U-RGB (Rtab [IRU], Gtab [IGU], Btab [IBU]) ; Y-V-RGB (Rtab [IRV], Gtab [IGV], Btab [IBV]) ; So this is the method of transforming the object coordinates to colors. In displaying from the input file to the computer screen the respective icon of an exactly defined shape and of an exactly defined color is assigned to the object according to attribute"A"in the data file for displaying the digital terrain model, whereby, at least two exactly defined object position pixels 3 of said icon are filled with color which color corresponds to their position in the XY plane of the geographical system of coordinates so as it is shown in Figure 1. At least two pixels because it is suitable to complete this information with one or more control pixels. This redundancy is introduced mainly to verify correctness of the inputted information in case of a back- question about the object displayed.

Pieces of data on altitude value together with the X-, Y-coordinates are transformed into coordinates of the displaying surface and in the displayed cross section they form vertices of a polygon. This polygon is drawn in cross sectional cuts beginning with the largest distance from the actual viewer position, whereby, said pieces of data on altitude together with said X-, Y- coordinates are transformed into coordinates of the displaying surface and in the displayed cross sectional cut they form vertices of a polygon, which polygon is filled with a color, whereby, the color shade is darker with the increasing distance and lighter with decreasing distance to the actual viewer position. Thereby, certain plasticity of the image is achieved. A similar principle is used in drawing an icon, which icon characterizes the geographic forest layer. With increasing distance from the actual position of a viewer the icons get more blue tinge, nearer to the actual position they have a green shade. Thereby, plasticity of the image is even more enhanced.

The so drawn terrain model is prepared to accept questions asked by a user.

The user determines the object of interest on the basis of visual recognition of the icon shape and the icon color. The user places cursor of the positioning device on the icon-icon pixels 1, so as it is shown in Figure 1 and presses the respective button of the positioning device. Color of the pixel indicated by cursor is read. If the color characterizing certain object is recognized, the icon shape is known and according to this known icon shape the upper pixels of the colored icon field are searched for-upper pixels 2 so as shown in Figure 1. If said upper pixels are recognized on the basis of their color and shape, position of the pixels carrying the information on object position is also known-pixels 3. Color of these pixels is read, separated into its brightness components, indicators of the brightness components are determined from the tables of the brightness components and using the indicators the numerical value of the components is reestablished in the plane of the displaying surface. By a transformation inverse to that in the drawing up, the original coordinates in the plane of the geographical coordinate system are obtained. On the basis of the so reconstructed coordinates it is possible to obtain a detailed information on an object in a database of the respective digital model of territory.

After obtaining the accurate coordinates various operations can be carried out above the terrain model, e. g. to determine the distance and direction to an object, to determine relief from an actual position to the specified object, to determine optical visibility of an object from various directions, etc.

Industrial Use The method of making interactive digital terrain model will find direct use in making digital relief models, when their points array forms a regular square grid. A point array forming an irregular, triangular or square grid requires to be transferred to this regular square grid. The method of making interactive digital terrain model finds use especially in digital models of medium and small scale (1: 200,000,1: 500,000). Such models are used in pre-flying training of pilots, in terrain analysis of larger areas, etc., using the common computer equipment.