Surveying property lines with RTK GPS

There are many brands of RTK GPS for land surveyors.  Trimble, Topcon, Leica and spectra precision are just a few brands that come to mind.  I have used both Trimble and Topcon RTK GPS and both have their advantages/disadvantages.  Trimble and Topcon both have their own data collector software.  Trimble uses Trimble Access which integrates surveying equipment with office software easily from the field.  It makes it possible to upload and download from and to the data collector, from the office.  This saves time by keeping your crews files up to date and allowing access to collected data almost immediately.  Trimble gives the user plenty of control in the field to manipulate coordinates with localizations.  They are simple to do and work very well.  If you make a mistake it is as simple as deleting the localization and starting over.  There is no need to take new shots on the local points.  Topcon definitely lacks in localizations.  Trimble also makes it simple to hold one point fixed at any time in the survey and Topcon once again makes this very difficult.  With topcon it looks like it will be simple to do this task.  All that is required is to start the base at a point autonomously and then with the rover tie in a known point.  At this known point you store the point as a check point and simply hit the correct base button.  This works very well, except that the coordinate for the base does not change to reflect the correction.  Most of the time it makes no difference, but if you try setting on this base with a total station and do some more surveying.  Or if you use this base point as a back sight reference for the total station you will quickly see that it does not work.

Topcon’s RTK is a less expensive option compared to Trimble.  This is probably due to the lack of money put into their software known as TopSurv.  It does look like Topcon is working on their software as there are updates every 6 months or so.  I have had great luck with initializations using Topcon RTK.  It works nice in tree cover and around buildings.  Sometimes I hear people complain about how bad their GPS is working.  They tell me that their rover won’t get initialized and when it does it doesn’t hold the fix well.  Its funny when  they blame it on the GPS and I can see their base under a tree or beside a large building.  I let them know that the sky view at the base is extremely important.  If you cut off even just one or two satellites at the base it will make it very difficult to get initialized at the rover, especially if there is any multipath from trees and buildings.

The thing I like about surveying property lines using RTK GPS is that you don’t need inter-visibility like we needed when using a total station.  This makes it possible to mark a property line without cutting down a bunch of trees.  We usually just place wooded stakes on line along the property line at spots where the GPS works the best.  As the stakes are Old Iron barbeing placed it is then possible to use a chain saw or axe to trim branches along the line through the bush and place any marks on the tress that are required.  The stakes are mainly used as a temporary mark for us to trim and mark the line and we typically remove the stakes as we go.  If there is nothing to mark or the line is difficult to spot we will leave the stakes and paint/flag them so the owner will see it.  In the city property lines are a bit different.  Most people in the city don’t like when you start cutting stuff with a chainsaw, so I will typically just leave it in the truck.  Stakes are placed along the property line and if it is not possible to get initialized we will just place offset stakes and use a tape measure to place the stake at its proper location.  Iron pins are place at the corners of the property.  This sometimes is impossible to do using GPS.  We would then place 3 points with the RTK and set on one with the total station.  Once the total station is set up we will back-sight one of the other two points. Using the data collector we will turn to the location for the iron stake and place it and tie it in.  Then I back-sight on the remaining nail and turn once again to the iron stake that we placed and take a check shot on it.  This will confirm that our control points are good.

If you plan on placing control points in the ground using GPS to use with a total station, make sure that the distance between the points is sufficient.  I will typically try to get these points 200 metres ( 650′ ) apart, if possible.  If it is impossible to do this and I am only tying in a few points I will make sure my back-sight will be double the distance to the fore-sight. If you need to use these GPS points to traverse a long distance I would place them farther apart.  The accuracy of a point placed with GPS is only within about an inch or two.  If you placed your control 200 metres apart and traversed 400 metres your new point could be out as much as 8 inches.  RTK is not that great to set control to be used with a conventional instrument especially if you need to traverse a long distance.  I would recommend placing two points farther apart and using static GPS to tie the control in.  If you can only see a short distance due to a road being curved or rolling in such a way as to limit visibility.  Place two points farther away and don’t worry about inter-visibility.  Simply traverse between the points using the total station.  The back-sight might not be as long as you would like, but it is better than if you just tie these two points in the RTK GPS.


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