Ashtech Z-Surveyor Service Manual download pdf (Page 13)

Introduction 5

Introduction

of one or more known points. If the survey included any redundant observations

(closed loops, repeat observations), then a least-squares adjustment can be

performed to help identify any blunders in the data and improve on the final point

positions. The capability to perform a least-squares adjustment is included with

the post-processing software package.

Applications

Post-process GPS is well suited for most surveying tasks. Systems are being

used today for control establishment, boundary surveys, and mapping

applications such as topographic, planimetric, and as-built surveys. Post-process

GPS is also very efficient for volumetric measurements such as stockpiles and

gravel pits.

In many cases, a GPS system will be vastly more productive in these types of

surveys than a conventional total station, with the added benefit that a GPS

system can be operated by only one person.

Limitations

GPS surveying systems have limitations that affect their ability to perform some of

the survey tasks discussed above. Being aware of these limitations will ensure

successful results from your GPS surveys.

The main limitation is not confined to post-process GPS but is a limitation of the

GPS system in general. As discussed earlier, GPS depends on reception of radio

signals transmitted by satellites approximately 20,000 km from earth. Being of

relatively high frequency and low power, these signals are not very effective at

penetrating objects that may obstruct the line-of-sight between the satellites and

the GPS receiver. Virtually any object that lies in the path between the GPS

receiver and the satellites will be detrimental to the operation of the system.

Some objects, such as buildings, can completely block the satellite signals.

Therefore, GPS can not be used indoors. For the same reason, GPS cannot be

used in tunnels or under water. Other objects such as trees can partially obstruct

or reflect/refract the signal, so that reception of GPS signals is 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. Therefore, GPS is not effective in a forest.

This is not to say that GPS surveying systems can only be used in areas with

wide-open view of the sky. GPS can be used effectively and accurately in partially

obstructed areas. The trick is to be able to observe, at any given time, enough

satellites to accurately and reliably compute a position. At any given time and

630045.book Page 5 Sunday, February 11, 2001 11:38 AM

1 2 ... 8 9 10 11 12 13 14 15 16 17 18 ... 57 58

No comments

Ashtech Z-Surveyor Service Manual Page 13