236 
- Individual weighting of the X, Y and Z control point coordinates 
- Weighting of measured photo points 
- Weighting (and thus retention) of the exterior orientation parameters 
- Interior orientation if the observed points are distributed appropria- 
tely | 
- Use of control points in local coordinate systems 
- Use of special control elements such as distances, coordinate differen- 
ces, horizontal and vertical angles 
- Introduction of the stereo camera condition 
- Output of standardized corrections and of the inherent reliability 
for gross error detection. 
2.3.3 Elevation Measurement 
  
In many applications, profiles or grids are nowadays used for terrain ele- 
vation measurement instead of direct contour line measurement for economic 
reasons. Static grid measurement with analytical stereoplotters was found 
to be the fastest and most reliable method. This method consists in auto- 
matically prepositioning the floating mark in plan to the grid points in 
the model so that the operator only has to set the elevation and initiate 
recording. Various investigations of the attainable precision have shown 
that the point density and the terrain configuration have a decisive in- 
fluence apart from the measuring precision. In order to be optimal also 
with uneven terrain, an optimum grid measurement method should allow the 
grid mesh to be matched automatically to the local terrain conditions with 
the point distance being specified by an objective criterion. In 1972 al- 
ready Makarovic has proposed an interactive acquisition method under the 
name "Progressive Sampling" which satisfies these requirements to a large 
extent /6/. The Institut für Photogrammetrie und Ingenieurvermessung (Prof. 
Konecny) of the University of Hannover has implemented this method with am 
analytical plotter for the first time /7/. 
With the optional B75 PROGRESSIVE SAMPLING program, a program developed by 
the Lehrstuhl für Photogrammetrie (Prof. Ebner) of the Technical University 
of Munich is now generally availahle which can be said to be optimum in 
view of the practical experience that has been integrated /8/. 
Departing from an initial wide-mesh grid and further information such as 
break lines,.approximate.local.terrain curvature data is computed by means 
of second derivatives. The curvature data is used to decide whether the 
initial grid should be densified locally by halving the grid point distan- 
ces. This procedure is iterative, i. e. all additionally measured grid 
points are analyzed again during the next pass and densification occurs 
again, if required, until a lower threshold is reached (Fig. 2). In the 
Planicomp, automatic positioning of the densification points and consecu- 
tive computation of the second derivatives is so fast that the operator 
does not detect any delay and can concentrate on elevation measurement. 
Depending on the terrain morphology and the desired DEM quality, progres- 
sive sampling allows the measurement time and the number of measured 
points to be reduced by 30 to 70 %.