Accurate Handheld GPS point marker

[arcade_007]

I'm looking for some info on a handheld GPS unit which can do the following:

Mobile unit with accurate GPS, ability to mark specific points (up to 50 points), need to be able to transfer this data to google maps or earth and have the points marked on a map (both map an satellite view)

The GPS coordinates must be very accurate within a few centimeters.

does anyone have suggestions

[Tiny]

Not possible in Australia, with civilian grade GPS, as we have at best a 3 metre inaccuracy on Civilian GPS.

In some other countries they augment that with for an inaccuracy of 1 metre.

So within centimetres is unlikely.

As for uploading waypoints, tracks & routes then viewing in google earth, that has been available for many years with Garmin GPS.
I've been doing that with my Garmin Etrex Vista for 7 years ore more.

First I load the Tracks or Waypoints on my PC then open in MapSource or BaseCamp, then view them on my maps.
Select view in Google earth & up they come in GE, you can also print the map view or send them to other people.

[arcade_007]

Hi Tiny
thanks for your reply. So what do engineers, surveyors etc use to pin point locations accurately. how accurate is the garmin you use.

[Tiny]

The best accuracy as shown in the accuracy field on my Garmin is 3 metres.
EDIT; forgot to mention, my GPS is WAAS cappable so in the USA I would get around 1 metre if in a WAAS area.

What a surveyor uses; is very expensive equipment combined with comparison & error correction techniques.
See below, quoted from;

Surveying with GPS

Initially developed for military use, GPS is now part of everyday life; used in mobile phones, in-car navigation and search and rescue equipment to mention just a few. But there is a wide variety of equipment and techniques that can be used for surveying.
This web site provided by the Smithsonian National Air and Space Museum gives more detail .
GPS was rapidly adapted for surveying, as it can give a position (Latitude, Longitude and Height) directly, without the need to measure angles and distances between intermediate points. Survey control could now be established almost anywhere and it was only necessary to have a clear view of the sky so the signal from the GPS satellites could be received clearly.
GPS is similar in some ways to the previously discussed, except that the known positions are now the GPS satellites (and their orbits) 20,000 km in space. The equipment and calculations are extremely complex, but for the user the process is generally very simple.
In the commonly available receivers, the GPS receiver almost instantly works out its position (Latitude, Longitude and Height) with an uncertainty of a few metres, from the data broadcast by the satellites. This data includes a description of the satellites changing position (its orbit) and the time the data was transmitted.

Figure 22: GPS Point Positioning.

Figure 23: Geodetic GPS Receivers

GPS Baseline
The GPS receivers used for surveying are generally more complex and expensive than those used in everyday life. They use the two frequencies broadcast by the GPS satellites and they use the physical characteristic of the GPS signal (the phase) and sophisticated calculation methods to greatly improve the accuracy of the positions obtained. These receivers usually have a separate high-quality antenna.
A GPS baseline uses two survey-quality GPS receivers one at each end of the line to be measured . They collect data from the same GPS satellites at the same time. The duration of these simultaneous observations varies with the length of the line and the accuracy needed, but is typically an hour or more. When the data from both points is later combined, the difference in position (Latitude, Longitude and Height) between the two points is calculated with special software. Many of the uncertainties of GPS positioning are minimized in these calculations because the differences between the observations at each end of the baseline are used.
The accuracy obtained from this method depends on the duration of the observations, but is typically about 1 part per million (1 millimetre per kilometre) so a difference in position can be measured over 30 kilometres with an uncertainty of about 30 mm, or about 100 mm over a 100 kilometres. Because the GPS satellites are in a very high orbit (20,000 km) the ends of the GPS baseline can be hundreds, or even thousands of kilometres apart and still observe the same satellites.
Although a single baseline from a known position is enough to give the position at the other end of the baseline, additional GPS baselines to other points are often measured to give a check on the results and a an estimate of the uncertainty of the calculated position.

Figure 24: GPS Baseline Measurement

Kinematic GPS
There are many variations on this type of GPS surveying, but basically it is similar to the GPS baseline method, except that while one GPS receiver remains on a known position (Base Station), the other moves between points and it only needs to be at each point for a few seconds. Corrections to the GPS data (based on the known Base Station position and its position computed from the GPS) may be immediately transmitted from the receiver on the Base Station to the receiver at the other end of the line (the remote station). The position of the remote station can then be computed and stored, all within a few seconds. Radios or mobile phones can be used to transmit the corrections. Although this method can give similar accuracy to the baseline method previously described, to do so this method is generally limited to a distance of about 20 kilometres.

Figure 25: GPS Real-Time Baseline Measurements