USA-4 PHOTOGRAMMETRIC ENGINEERING 
grammetrists assigned to the Geodesy Division when higher-order triangulation 
is established in a map area. The photogrammetrist identifies the stations on 
the photographs, and selects intersection stations to aid in subsequent photo- 
grammetric triangulation, as well as accepting any assignment that might aid in 
the progress of the geodetic operation. 
When the stockpile of a given chart has become exhausted and reprinting 
is considered, the problems of revision become important [4]. Usually, consider- 
able data are already on hand indicating needed revisions, such as a new pier, a 
shipwreck, a navigation facility removed, or a reduced water depth through 
sedimentation. But the chart compiler cannot be certain, ordinarily, whether 
his data are complete. One method for determining the amount of added re- 
visions of the land information needed on a given chart is to take aerial photo- 
graphs of the coastline. A comparison of the photographs with the chart indi- 
cates the extent of revision needed. If the revisions are extensive, the chart may 
require reconstruction, which is equivalent essentially to making a new map. If 
the revisions are minor, they may be applied to the chart negative by hand to 
agree with the photographs. In both instances, photogrammetry is utilized. 
If extensive revisions are necessary, the procedure is not much different 
from that for constructing a new chart in an unknown territory, such as Alaska. 
However, sufficient horizontal control may often be identified so that none may 
need to be established. If contours are required, levelling may be needed to es- 
tablish vertical control in areas required by the stereoscopic instruments. On 
the other hand, a very complete field inspection may be needed before compila- 
tion. Such areas are often well developed, whence the inspection is more diffi- 
cult than for a new chart. 
If the revisions are expected to be minor, a small airplane and crew may be 
assigned to photograph the shoreline from a relatively low altitude, such as 
5.000 feet, where the nautical chart scale is 1:40,000. The navigator, who is well 
versed in the over-all chart requirements, may pilot the airplane with a con- 
noted nautical chart in his lap, marking the places that appear different than 
shown; and then reflying the place to take at least three photographs with 
stereoscopic overlap. This was done for the entire eastern coast of the United 
States in two seasons, using only three or four days per month when both the 
sun and tide were in correct relationship for photography at low-water stage. 
Contact prints of the aerial photographs are ideal for graphic compilation 
of revisions, as small areas near the centers of the photographs are used almost 
entirely. Sufficient sharp detail, common to the chart and to the photograph, 
exists so that the areas need only to be matched and then applied directly to 
an appropriate copy of the chart. 
The present method consists of delineating a change on a contact print in 
a bright ink, using a stereoscope, and transferring the photographic detail 
(1:10,000) onto an appropriate copy of the chart (1:40,000) using a Focalmatic 
desk projector. Larger revision areas are compiled on a stereoscopic instrument 
and then applied to the chart. A Stereotop has been purchased recently to facili- 
tate this work. Other phases and variations of this operation, and the use of 
plastic scribing, are included in [4]. 
Aero-triangulation is applied in four different ways: (1) nine-lens, graphic, 
radial plotting, (2) Multiplex bridging, (3) model templet bridging with the 
Kelsh plotter, and (4) stereoplanigraph bridging. 
For the nine-lens mapping, the photographs have 70 per cent forward lap 
and 60 per cent side lap which establishes a geometrically strong triangulation 
solution. The transforming technique has now been perfected to the place where 
junction errors are normally no greater than 0.1 mm near the center and 0.2 at 
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