Specifications

For the same reason, GPS cannot be used in tunnels or under

water. Other objects such as trees will partially obstruct or re-

flect/refract the signal; reception of GPS signals is thus very

difficult in a heavily forested area.

In some cases, enough signal can be observed to compute a

rough position. But in virtually every case, the signal is not

clean enough to produce centimeter-level positions. There-

fore, GPS is not effective in the forest.

This is not to say that your ProMark3 surveying system can

only be used in areas with wide-open view of the sky. GPS can

be used effectively and accurately in partially obstructed ar

eas. The trick is to be able to observe, at any given time,

enough satellites to accurately and reliably compute a posi

tion.

At any given time and location, 7-10 GPS satellites may be

visible and available for use. The GPS system does not require

this many satellites to function. Accurate and reliable posi-

tions can be determined with 5 satellites properly distributed

throughout the sky. Therefore, an obstructed location can be

surveyed if at least 5 satellites can be observed. This makes

GPS use possible along a tree line or against the face of a

building but only if that location leaves enough of the sky open

to allow the system to observe at least 5 satellites.

For the above reasons, make every effort to locate new points

to be established in areas where obstructions are at a mini-

mum. Unfortunately, the site location is not always flexible.