United States 
of America 
1 
   
- 69. 
  
  
Ground Control Points. 
Ad 5.21 
rules proposed are in the "Propos 
for a new Swedish Surveying Ordin 
co", 
Four points of known vertical ani 
horizontal positions are required 
in each photopair. In large scale| 
mapping projects, 1:25 000 or lar 
ger, the vertical positions of i 
points in each model are usually | 
established by field surveys. The 
horizontal positions of the four 
points would be established by 
aerotriangulation procedures base 
on approximately one field establi 
ed position for esch 5 models. | 
At scales of 1:50 000, the horizg Union 
tal and vertical passpoints would soc. . 
be established by aerotriangulatio 
Basic ground control requirements 
for these map scales will vary wii 
each project. On the average, bani 
of horizontal and vertical control 
which are laid out perpendicular | 
to the flight lines at about 15 | 
model spacing are used. Airborne l| 
rizontal control established by 
HIRAN and vertical control by Ter 
Profile Recordings (radar) signifi 
cantly reduce ground surveyed cont! 
requirements. | 
Basic ground control is establishe 
by ground survey techniques, and | 
airborne trilateration techniques 
(HIRAN line crossings). All horiszd 
tal control is positioned in the | 
: R Al 
Universal Transverse Mercator Grid 
| 
I