Full text: XVIIIth Congress (Part B1)

  
Frame ground coverage (GW) = 1760/0.4 = 4400 feet 
Recommended average flying height (H) = 0.5 GW/tan20 = 6044 
feet 
Thirty percent side lap between alternate flights ^ 0.3 x 4400 — 
1320.0 feet 
Pixel ground resolution = 4400/2000 = 2.2 feet 
B/H = 1760/6044 = 0.3 
In order to have a priori estimate for the contour interval possible 
with the digital camera at the calculated parameters the following 
photogrammetric criteria were utilized (Light, 1990) 
Piel size 5 HKxB/H 10; bom noiimmaih-datinto A Round (1) 
where: 
K = non-dimensional number expressing the degree to which 
Stereocorrelation can be achieved. (Typical bounds for K are 
0.2<K<1.5) 
0, = precision of a single height observation which is assumed to 
be one third of the contour interval (CI) (0, = 1/3(CI)). 
B/H = the base to height ratio. 
Substituting B/H=0.3 and K=0.3 (or the parallax errors=0.3 pixel 
size) in equation (1) will result in the following value for CI. 
Pixel size = 1/0.3x 03x03 x CI = 
or CI 4 2.272032 73'- 22m 
This figure served only as a priori estimate but the possible 
contour interval should exceed the expectation. 
2.3 Data Acquisition 
The digital aerial photo was acquired during May 1995. The 
project area is located at Desloge, Missouri and it is characterized 
by variation of terrain patterns from the very steep slopes along 
the river banks to rolling and flat areas. The digital frame camera 
was installed on board the Photo Science, Inc. Piper Navajo 
Chieftain plane. Four flight lines and a supplementary line were 
flown at about a 6500 foot flying height which resulted in a slight 
increase in endlap and sidelap coverage. The data was then 
processed at the ground facility of Daedalus Enterprise, Inc. at 
Ann Arbor, MI, where about 100 black and white digital images 
were obtained in a TIFF format ready for the softcopy 
workstation processing. A few days prior to the flying, the 
ground surveying crew established an accurate network of nine 
control points using GPS techniques and the already established 
NGS points. Each ground control point was marked with an 
aerial photography target in the shape of two crossed panels one 
and a half feet in width and six feet in length. 
2 
2.4 Digital Photo Triangulation 
A total of 24 frames that covered the testing site and a total of 
eight paneled ground control points were used to form the block 
of photos that is later oriented and triangulated on the 
Autometric-Vision International softplotter. Considering all the 
problems that are inherent in the interior parameters of the DFC, 
the stability of the triangulated block was of concern. Therefore, 
a smaller block of seven frames with a suitable number of ground 
controls was used for the purpose of this analysis. 
The interior orientation was achieved by measuring the pixel of 
the four corners and the image coordinates system origin was 
assumed to be at the point of intersection of the two axial lines 
connecting the opposite corners. Although this assumption is not 
free from error, it was necessary as no accurate information was 
available about the principal point location. 
2.5 Image matching autocorrelation, and surface modeling 
The triangulated pairs were then correlated and a digital elevation 
model was generated using the available tools of the softplotter. 
The generated surface was then modeled using both the terra 
model and the Intergraph MGE Modeler. A set of two foot 
contours was generated. Figure 1 shows a clip from the generated 
contours. 
3. RESULTS AND DISCUSSION 
In examining the pixel resolution on the softcopy station, the 0.7 
m pixel ground coverage seems to be adequate for revealing 
ground details. The ground targets were easily distinguishable. 
Due to an appreciable amount of error propagation in the larger 
block which resulted from camera interior instability, number and 
distribution of control points (as some of the control points 
disappeared prior to flying due to local land use activity), a 
smaller block of seven digital photographs was triangulated and 
autocorrelated to produce the digital elevation model (DEM). 
Examining the obtained DEM and the derived contours reveals 
a great agreement with the set of contours derived conventional ly 
from a photogrammetric stereoplotter. However, it is found that 
there is a bias in the digital surface generated from softcopy 
which is clearly reflected in Table 1. The table shows the data 
from twenty points selected on a variety of terrain patterns that 
were measured stereoscopically from the three-dimensional 
digital stereopair displayed in the softplotter and the same 
locations were selected and measured in the surface generated by 
conventional photogrammetry. It is noticed that discrepancy is 
reduced in magnitude as the evaluated point is closer to a control 
point. Some other points deviated from the root mean squares of 
error or bias and that is believed to be either because of the 
changes in the ground surface as there were a few years between 
the two mapping dates, or it is because of some trees or brush 
covering the ground surface. 
It is also noticed that the discrepancy pattern changes 
considerably at the step slope at the river banks as the correlation 
technique faces more ambiguities due to slope relief and dense 
tree coverage. 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B1. Vienna 1996 
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