Questions for the GPS savey

[dangerranger]

I have no experiance with a GPS , the county that I work for is building a new complex.the contractors are mapping the underground valves, boxes, and pipes with a GPS.Its my understanding that GPSs have a built in error to keep them from being used as a wepon sight. Does this mean that I need to use the same Gps that was used to map?or will another of the same model work?Or is the error so small that it wont make any differance? Any oppinions are welcomed.

[Siskiyou]

Suggest that you check out the following site http://www.fs.fed.us/database/gps/ This site should tell you a lot about the two different types of gps units.  Basically the type used for surveying is more costly, and more accurate.  A true surveyor uses a computer program which corrects his data.  

I take part in a sport called geocaching which you can find at geocaching.com  Yesterday after pulling the Lat/Long for a cache the size of a 5x7x2 Tupperware contain I drove 30 miles.  Got out of my vehicle, hike a distance over snow and found the hidden container.  I was using a handheld Garmin Legend gps.  Once I got to the site I had to look for half an hour for the item which was buried in snow.  I consider the Legend to be a darn good recreational gps.   If I was looking a better recreational gps I would buy a Garmin GPS Map76.  It cost more but you can also plug an external antenna into it.  This is important because tall timber and tall buildings block gps signals.

If the gps unit is important to your pay check find out what gps unit the original surveyor used and what software.  Will his data be available to you.  GPS software is proprietary.  Brand A software will not upload to Brand B gps units. Vendors will tell you that their software is compatable with Brand A & B.  But check it out, normally only downloads from the gps unit are compatable, not uploads.

There are times when I am out hiking and I set my gps to create a track.  The same method to be used if I was walking out a survey line, pipeline, ect.  When I walk down a road on the on the Northside a track shows.  When I return on the Southside a track shows on that side.  If tall trees block the sky I may loss the signal.  That where a good external antenna comes in.  My boat gps unit has an external antenna and it receives very good when parked up against the house and under heavy oak canopy.

[Will Bison]

The GPS your contractor is using probably has centimeter accuracy and many of the survey companys I contract with use them. They are VERY expensive.

You probably want a simple handheld GPS which will give you an accuracy of 3-6 meters. In other words, imagine you are in a bubble with a radius of 3-6 meters/yards.

For most of us that is plenty good.

Selective Availability was turned off in May of 2000. The US Govt can mask any area of the World they want and thus preempt the use as a weapon by "bad guys".

Not knowing how accurate you need to be, most of the mid-range handhelds do a passable job and tend to be more accurate than topo maps can be printed.

Hope this help you.

[dangerranger]

Thanks for the info. Now I have more questions for the contractor than ever.With the Geo catching example, if you had been using the same model and brand would the info have taken you to an exact spot rather than looking for another 1/2 an hour?Or would you have to use the exact GPS to get to that exact spot? Or will they not lead you to an exact spot?As this is a large project I stand a good chance of getting the contractor to supply us with a GPS if I act early,and know exactly what I want. with respect to the size of the target, it can be anything from the size of a manhole cover to a valve handwheel[8 to 10"] and several feet underground.

[jackfish]

There are now handheld units that achieve submeter accuracy that are not that expensive considering what they can do.  The Trimble GeoExplorer XT is around $2000.  It is a PocketPC that runs the Windows Mobile 2003 operating system and will also run Geographic Information Systems software.  Makes that Garmin GPS V look paltry.  But they are more for field data collection than sportman use.  It would be great to have topo maps and aerial photos to use with a submeter GPS though.  Soon, handheld units for the sportsman will be more like the GeoExplorer.

http://trl.trimble.com/dscgi/ds.py/Get/File-128927

[Siskiyou]

I am getting ready to stash a cache in the next couple of weeks.  My gps unit does not "Average" it's reading.  To over come that so the best Lat/Long are published on the geocaching website I will take a number of reading(7 to 10) and take the average.  I will take a reading at the cache and write it down.  I will then walk away and return to the cache and write down that reading.  The average reading will be used.  It works in most cases.  Published reading(locations) for caches are taken by all brands of gps units.  A lot depends on the operator.

When I am looking for a cache I use a number of tools.  One of those tools is topo software.  When I find a cache on the net that I want to look for I print out the info.  I then enter the infor(Lat/Long) into my gps as a waypoint.  I then connect the gps to my computer and download the waypoint to my MapSource Topo software.  I am fairly good at reading topo maps and finding my way in the woods.  Using this information I hope to find the cache.  In your case in would be the project plans.  Do I always have a map?  Nope by it helps on the more complex caches.

Depending on the Satellites you may recieve more Satellites at a given location at different times.  The assumption being the more Satellites the more accurate reading.

There is a learning curve in getting the most out of a gps.  Some folks want just a Lat/Long but others want more.  I ran into a fireman last year in Montana who was going to all the structures in his district to  record location for his dispatch center.  This is very important inportant in rual area with one or two volunter stations covering a hundred square miles.

The fishfinder gps in my boat is a Huminbird with an external antenna.  Side-by-Side with my Garmin Legend they have the same reading.

If you can get a contractor provided gps with the same software that is the way to go.  If repeatability is important do not use a recreational gps.  I did a test on local survey pins.  I created waypoints at five pins.  I then retraced my course over a few weeks as a learning process.  What I learned was that the location kept moving a few feet.  

The other day I was looking for a 5x7 inch container buried in the snow.  It was tough.  Part of the problem was that I was in tall timber.  It took about 30 minutes to locate the item when I was within a few feet.

[dangerranger]

Thanks for all the help. I've been reading a ton of data , but the first hand info really helps put it all together.Im going to try to get the engineering firm to include a GPS and soft ware with there contract. In the size of this contract this should be a very minor cost. I also looked into Geo caching sites. I found someone near my home that is going to help me learn a little more about hand held gps and mapping software. It looks like fun. Thanks for your help.

[PA-Joe]

Even using the same GPS unit will not help. It depends on the exact location of the satellites on the day the readings are taken. No two days or time of days are the same. The $2,000+ units will provide 1 meter accuracy when the data is calibrated. $10k units get cm accuracy. Make certain that your contractor take several readings at known locations (benchmarks) and then have those locations field confirmed by a real (true) survey. You can then calibrate the data to those know benchmarks. Another option is to set up a master base station or benchmark using one GPS unit that runs 24/7. You can then calibrate the other GPS units to that base station. Here in the USA most states have a master base station or two. We calibrate our units to the master base stations. There may already be one near you.