Land Survey in Romania

Geodetic survey

Here we have identified the best in differential mode GPS available for this task - the Ashtech Z-Surveyor dual frequency, 12 channel, differential mode GPS. This is a successor the well known P12 and Z12 systems.

This dual frequency system compensates for ionosphere irregularities. It also embodies two key features which mark it out as the market leader:

Ashtech use a patented tracking receiver (Z-tracking) which provides a 13 dB signal-to-noise advantage over the more normal cross-correlation receivers. This, in effect, creates a much more powerful antenna and so reduces signal ambiguities - and hence spatial ambiguities - over other, competitive systems.

The second differentiating feature is the use of a carefully tuned antenna. This technology maintains a phase centre stability of better than 1 mm in the presence of adverse signal conditions.

Topographic survey

The requirements for small and medium scale topographic survey are not as stringent as those for geodetic survey. However, convenience and speed of operation are crucial.

Our solution here is to use a satellite based differential system (DGPS). This works in conjunction with a reference signal transmitted in the L1 microwave band. The choice of L1 is a compromise between precision and ionospheric disturbance.

The use of a DGPS provides suitable precision, but without the logistical restrictions of true differential operation.

Our chosen equipment is the LandStar DGPS from Racal which provides sub-metre RTCM accuracy.

Cadastral survey

Cadastral survey requires high accuracy performance but, in general, over shorter baselines. For long baselines, clearly, the Z-Surveyor (above) would be used. But there is an equally good implementation for shorter (up to 15 Km) baselines based on the Russian GLONASS system. This is the Ashtech GG-Surveyor that combines both a GPS receiver and a GLONASS receiver.

The combination of both GPS and GLONASS makes this unit ideal for use in the urban areas as the dual system provides double the number of channels than are available from GPS alone.

Using the GG-Surveyor in RTK (kinematic) mode, provides 1 cm accuracy (RMS).

Site survey

Modern site survey is dominated by the use of Total Stations. There are many such equipments available on the market and it is only the specifications and quality of manufacture that separate them - also the price one has to pay!

The key components of a Total Station that affect its performance are -

The quality of the optics

The quality of the digital shaft (angular) encoders

The power of the laser. This is particularly critical for instruments which are likely to be used in hot climates as the more powerful light beams are somewhat less susceptible to shimmer (scintillation).

Taking all things into account, Geo Strategies uses Sokkia SET Total Stations as these use Ashai optics associated with a high power transmitter. In addition, these units have dual-axis compensation which overcomes many of the problems normally associated with site survey.

GIS data collection

For GIS data collection, we use any or all of the above equipments according to the accuracy required.

For many purposes the Racal Landstar provides an ideal solution. This man-pack equipment provides instantaneous 1 metre accuracy which is frequently adequate for recording purposes - for determining the position of electricity pylons, street lights, forest boundaries and similar.

In those situations where sub-metre accuracy is required, we use the Magellan ProMARK X-CM. This is a hand held 10 channel receiver which can provide 15 mm accuracy with longer occupation times. It is particularly attractive for field work due to its low weight and high capacity data storage.

Post processing

Key to high accuracy survey using GPS or GLONASS technology is post-processing. This comes at three levels, based on -

Real time differential data

Pre-calculated orbital data

Precise ephemeris information (post calculated from IGS data)

Many software systems provide facilities for this type of processing. After extensive research, we have decided that Terrasat post processing from Spectra Precision provides the widest range of facilities and capabilities.

Full deails of our Survey services are shown elsewhere.

GPS ephemeris

Precise ephemeris metadata