D 10.0
7.0 5
Figure 2. The result of the DTM generation
The basic parameters of DTM are the distance between
centres of the nodes of the grid in the ground coordinates
system and the accuracy of the determination of their
positions. The cost and the time of the coordinates
(X, Y,Z) creation is also important.
Building DTM from existing maps needs the contours
extraction from the raster image of the maps registered
by a scanner. The contours are pointed by the operator
on the display screen. The same operation can be done
directly on the map using a digitizer. In this two cases
we receive the vector image of the contours in the
ground coordinate system. To obtain DTM, the heights of
the terrain, which are written as contours, should be
interpolate into heights in the nodes of the given grid.
Each cell of this grid that does not have a corresponding
positioned vector element will have an interpolated Z-
value. The Z-value may be determined as the result of
the linear interpolation. For example the Z-value can be
determined from the line between the closest known
values on both sides of the cell. The Z-value that
corresponds to one of the eight linear solutions with the
greatest slope is assigned as the final cell value. The
resulting surface may be irregular. To obviate such
problems e.g. an automatic smoothing routine should be
done, but this will not be described in this paper.
DTM building by photogrammetric measurements needs
as input material a stereo pairs ( at least one).
DTM can be obtained by two ways:
e calculating the elements of the relative and absolute
orientations,
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B3. Vienna 1996
e direct calculating X, Y,Z coordinates in the ground
coordinate system using the project transformation.
In the first point it is necessary to know the parameters of
a camera and the function from pixel to image
coordinate system.
In the second point it is very important to have the high
precision of the measurements of the control points
coordinates in the pixel coordinate system as well as
their accurate identification in both images. The algorithm
of DTM building can be obtained e.g. on the basis of the
correlation method. It matches area from two images
taking into account the correlation coefficient. The
algorithm calculate it many times for surroundings of the
pixel (node in a grid of the left image). The area is
matched for the maximum value of this coefficient. The
algorithm searches the one (left) image along the nodes
of the given grid. These nodes are determined a priori.
The information from one iteration (image pyramid) about
matching is used in the next one. Because of this we can
make shorter the time of the calculation.
The Figure 2 is the result of the DTM, generated by
correlation method, after interpolation.
4 ERRORS IN DETERMINATION OF X,T,Z
COORDINATES IN DTM
To calculate the errors in determination of the X,Y,Z
coordinates in the process of DTM generation is very
important because they affect the errors of the
orthoimage. We will consider the case when we use the
interior and absolute orientations in computation of X,Y,Z
coordinates.
We use the formulas:
o o
X-X, --Z ^. )eB(T)
C k p
; à (81)
Y-Y, z-Z3- jen!
C k p
C
Z-2. = B(—)
p
where
X,Y,Z the ground coordinates,
x$,y$ the image coordinates of the horizontal left photo
X, Y,, Z, the translation vector
p°=x;-x, the difference of the abscissas of the
corresponding points on the horizontal left and right
photos
c, focal distance
B the base of the photo in the ground coordinate system
The mean square errors m, m,m, we obtain first
calculating the differentials of (31), e.g. the formula of d,
is as follows
630
Figure
where
We te
includ
dp’ wi
interic
Figure
The g
0.5m.
The c
errors
proce
variak
gener
and (
the de
result
exteri
Z coo
After
of ste
