IN"STEiJCTIO]SrS FOR MAKING PILOT BALLOON OBSERVATIONS 6 



by the latter method stabihty and rigidity are Hkely to be diminished. 

 In selecting the observation points on a roof having a central struc- 

 ture it is well to consider the prevailing wind direction for the station. 

 The observation point should be selected on the leeward side and as 

 far removed from the central structure as extent of roof will permit. 



5. Location of two-theodolite station. — The geographical location of 

 a double-theodohte station is not materially different from that of a 

 single-theodolite station. Low horizon and angular altitude of obstruc- 

 tions have a similar application in either case. A double-theodolite 

 station is provided with two theodolites, one at either end of a suitable 

 base line. The station at which the balloons are prepared and released 

 may be known as the "primary station," "home station," or "station 

 A." The second may be known as the "secondary station," "field 

 station," or "station B, C, etc." The system of base lines should be 

 carefully laid out and should radiate from the primary station. 



The major base Une should be about 2,000 meters or more in length 

 and nearly at right angles to the direction of prevailing wind for the 

 station. Minor base lines should be laid out as nearly as possible in 

 a direction that will afford the best possible results when the surface 

 wind direction is other than the prevailing direction. Two base lines 

 are sometimes sufficient, though three or more will afford a wider 

 choice in selecting the base line at the time of observation to give the 

 best results for the current wind conditions. A base line nearly 

 parallel to the wind movement aloft is to be avoided. 



6. The view along the base line from either station must be entirely 

 free from obstruction. Each station must be in plain sight of the other 

 in order to facilitate signaling the release of balloon at the primary 

 station and the disappearance of the balloon at either station. 



The angular elevation of obstructions at the secondary station, in 

 the direction of the primary station, along either side of the base line 

 must be low enough to prevent interference of line of sight upon the 

 balloon as it moves away from the primary station in strong winds. 

 Under ordinary conditions there will be little difficulty, but when 

 strong surface winds prevail, especially if blowing across the base line, 

 the change in azimuth angle will greatly exceed the change of eleva- 

 tion angle at the secondary station. Therefore the elevation angle at 

 the secondary station for the first few minutes will be low. 



7. The stations should be located at approximately the same 

 elevation above sea level. In case they are not, computation becomes 

 more or less complicated in certain instances. This matter is dis- 

 cussed in later paragraphs under "Computation." 



8. Alarking observation point. — The observation points, whether for 

 single or double-theodolite work should be marked permanently. If 

 the point selected is on the bare earth, an iron pipe about 3 feet long 

 may be driven into the ground until quite rigid. A wooden peg is 

 then set in the upper end of this pipe and a- small nail or tack in the 

 end of the peg marks the exact point of observation. If the position 

 is on a graveled roof, the point may be marked by setting a small bolt 

 or screw in cement over the point desired. Whenever it becomes 

 necessary to use a platform for observation, a nail may be driven into 

 the planking. Any method whereby the point is permanently marked 

 is acceptable. 



If the observation point is on the ground, it will be found convenient 

 in leveling the instrument to have foot blocks on which to set the 



