.ONTAL 
ECOND VERTICAL 
ORRECTION 
  
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|. A spatial resection is performed on the aerial photo that 
carries delineated detail using an X, Y digitizer and the 
appropriate ground control. The rotational elements and the 
Spatial coordinates of the perspective center are computed 
and stored for future use, in the computer under an approp- 
riate identification code. 
2. Delineated detail on the photograph is digitized with the 
X, Y digitizer, tagging the data with meaningful codes. 
3. The ground coordinate for each X, Y reading is computed 
using the resectional data for the photograph - the solution 
reflecting an approximate elevation. 
4. The horizontal coordinate for the point is superimposed on 
the DTM and an elevation is interpolated for that position. 
9. Using the back solution of the resection algorithm an X, Y 
value is computed in the system of coordinates of the camera. 
There will be a difference between the X, Y reading and the 
X, Y computed so a proportionate change is made to the hori- 
zontal coordinate. That position is again addressed to the 
DTM. 
6. After approximately three iterations, a true spatial coordi- 
nate of the point will have been computed, and stored away. 
/. At such time as one wishes to plot the feature in map form, 
the spatial data can be retrieved from computer storage, pos- 
sibly symbolized and plotted with an X, Y plotter in map form. 
The technique holds real promise for the US Forest Service since 
much of the inventory of the Forest resources has been developed 
by interpretation and delineation using aerial photographs. There 
has been a continuing problem in the past of transferring delinea- 
ted data from the field-used photos to maps since these photos 
are rarely compatible with the stereoplotters normally used in 
the mapping process. So much of the transfer has been done using 
approximate methods. 
The combination of space resection of a delineated aerial photo 
and the DTM not only offers an accurate way making the transfer 
of data from aerial photography but also provides a string of 
code-identified spatial coordinates that can be computer stored 
and retrieved for future plotting or manipulation. 
Engineering Applications of the DTM 
  
Whenever earth is to be moved, cross-sections are developed so 
that the volume of earth being moved can be determined in the gra- 
ding process thereby allowing one to compute the cost of the gra- 
A a A 
ding. 
It is believed that the DTM can be applied most effectively to en- 
gineering projects of this nature, where the cost of moving earth 
is an important consideration. More than likely, the allowable 
error of the quantity of earth being excavated should not be in 
ia