Sect_5.4.4

5.4.4 GPS Static Surveying: Field Procedures

FIELD OFFICE PROCEDURES

It cannot be overemphasised that data should be processed as soon as possible after the observation session in order to assure the quality of the survey at an early stage. As a prerequisite therefore, all data should be systematically catalogued and archived between observation sessions (if there is time), or at the end of the working day at the very latest. Many problems can be identified at this stage.

The following are some typical field office procedures:

Data handling tasks -- transfer of data from receiver to computer.
Data verification, backup and archiving in field office -- transmit raw data to head office?
Preliminary computation of baselines in field office.
Preliminary quality control procedures, such as the inspection of repeated baselines, loop closures, and evaluation of (incomplete) minimally constrained network.
If appropriate software available, minimally constrained network for entire campaign can be built up one session at a time.
Command and control of survey parties -- develop contingency plans for repeated observation sessions.
Oversee calibration and testing of field equipment.
Preparation of campaign report, and maintain ensure reporting to head office and/or the client.

Without data safely downloaded from the GPS receiver, the survey work should never be considered complete.

A few hints:

Download data "asap".
Follow procedures in the operator's manual.
Most GPS receivers have many hours of internal memory, so daily download is a reasonable routine.
Delete files from receiver memory when data download procedure has been verified.
Download to P.C. harddisk, then to floppy disks, then make backup copies.
Store backup disks separately.
Label and write-protect floppy diskettes.
Be ruthlessly systematic with diskette labelling convention.
Cross-reference booking sheets to data files.
Verify data download, for example check number and size of files, and process as quickly as possible.

It is often more economical to reobserve a session than to spend a lot of effort in the office trying to track down problems. This option can only be exercised if the field parties are still in the area and have not moved on to a different area, or have returned to their head office. This is therefore another good reason for insisting on some field office processing.

A common problem that may be identified at an early stage is incorrectly measured antenna heights. Confusion often results when antenna heights are extracted from field log sheets, and compared with heights entered into the receiver message file by the field party. (Further confusion can result with proprietary GPS software which have built-in offsets that are automatically applied for their own antennas, and the field and office staff do not fully understand or are aware of them.)

Guarding against the "ultimate fieldwork sins":

Power loss is the most common cause of GPS equipment failure.

always have back-up power supplies!

Cable problems are the next most common sources of failure.

keep them in good condition!

Incorrect operation of receiver.

field staff must be trained!

Antenna height reading error is probably the most common field error affecting GPS survey quality.

know the antenna phase centre, check and recheck height reading!

Are you on the correct station?

good reconnaissance helps, get evidence of occupation!

Data collection must be coordinated, only common data from a minimum of two sites can be processed.

good teamwork, well designed observation schedule and well trained field staff!

Loss of data after survey session ends.

use ruthlessly systematic data management procedures!

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