Jump to content

Gps Constant Obs Point

GEO*Trailblazer 1

Recommended Posts

This is just a though that I have been pondering for some time,and some of the questions here in Benchmark Hunting kinda hit around it.


Idea: Have a New Type Cache and Constant GPS observation points.

For the ones who seem to be more advanced and want to know answers to their Questions.

This would be fairly easy to implement and maintain for those who want to have a control point or want to make a control point somewhere near their residence or work.

It could be updated daily at pre determined intervals,and also by the cachers whom come by to find and log (THEIR READINGS)and coorealate these to the constant much as a COORS Station does only these would be ours.


I have seen the handhelds get within .001 of the RTK readings without a setup of hours.and the $$$$$$$$$$$$$$$$$$$$$$$$$

We all have the equiptment and technology right in front of us.


What do you think of the Idea?

Link to comment
have seen the handhelds get within .001 of the RTK readings without a setup of hours.and the $$$$$$$$$$$$$$$$$$$$$$$$$

We all have the equipment and technology right in front of us.


Not possible. If it were only that easy, there is no hand held receiver (except maybe survey grade) that can do that. What the hand held is giving you is very misleading. While you think its close, its far away for any precise work. If your handheld was matching his RTK, then he was doing exactly the same thing you were and was going to post process his positions to refine them. We often would go out and RTK without any control in the area. We would use a NGS bench mark (or set our own point) and go the single point position survey option in the GPS receiver. This would allow you to RTK after about 20 minutes of letting the base station determine a approximate fix. Once we did the field work and had a day or 2 of observation data on the Base Station we would send the RINEX Data (from base stations only) to OPUS but after the precise orbits were available and get a solution to the Precise position of the our control point(s) in state plane coordinates. Then entered this corrected position into the laptop software and all our field observations (that matched your handheld) were instantly recomputed in SP. There are many ways in which to use RTK, some are not concerned with a high accuracy and other are. It all depends on what your goal is.


The broadcast ephemeris is only accurate from 2 to 5 meters. This ephemeris is used by the GPS receiver in the field. The RTK is using GPS + on board site position to compute DGPS corrections and sent by radio/hone modem to the RTK rover. There is a lot more to this than meets the eye, if you see someone roving around with a RTK, there is a DGPS base station out there somewhere broadcasting DGPS corrections to that rover, they have it set on a control point, they could be using Lat and Lon, state plane or any other coordinate system they need to. Even with all this sophisticated equipment and software, it still may be up to meters off and you have to hit it again at a different time or from a different base station to be certain and then 2 cm is about all the acc you can expect. No surveyor in his right mind would trust any position from a hand held unit, they are not designed for that kind of accuracy, although many think there are.


A precise ephemeris is computed after the fact and has accuracies ranging from 20 to 30 cm available after 24 hours, 5-10 cm available after 8 days, and 3-5 cm available after 14 days. The precise ephemeris files are available for download from: http://www.ngs.noaa.gov/CORS/Data.html. Use a precise ephemeris whenever you can.


Ephemeris refers the the orbits of the satelites.


National CORS

Edited by elcamino
Link to comment

In a nutshell, you will never get survey-grade, or repeatable accuracy, with a recreational handheld GPSr. Period.


Surveyors and geodisists who use high-precision, survey grade instruments to obtain their sub-centimeter measurements do not rely on the instruments alone to provide the final and true position. Their equipment (GPSr) is only the first part of a sometimes lengthy process.


The $100-$500 GPSrs that we use for geocaching and benchmark hunting only have an accuracy of +/-10 meters at any given time. A WAAS enabled unit may have an accuracy of +/-5 meters. While it may be possible to generate a reading from a handheld GPSr that looks similar (on the screen) to that of a high-precision surveyed coordinate, the error remains in that handheld GPSr and its reading.


Also, the handheld GPSrs are highly influenced by external forces and anomalies, such as blocked or partial signals, reflected signals (multipath), signal degradation, and gravity, to name a few. High-dollar, survey grade instruments are equiped to deal with those anomalies, and the post-processing of the data is to reduce the errors incurred by them.


As far as the jargon, it is high-precision GPS survey-speak and part of the lengthy process used. I can only guess at which jargon you need defined but here are a few:


RTK = Real-time Kinetic - High precision equipment generally falls into one of two categories: Static systems and RTK systems. Static systems are stationary, set on a point from a few hours or days to many months to obtain true coodinates. RTK systems are 'on-the-fly' systems which can obtain reading within a few seconds to minutes. Most RTK systems work in conjunction with a static base station and are not as accurate as a long-term static system. The handhelds we use for geocaching and benchmark hunting would fall into the RTK category, and are the least accurate.


Post-processing - the mathematical calculations done to the GPSr readings to obtain a true (most probable) coordinate position.


RINEX - a GPS data format


OPUS = Online Positioning User Sevice - A data processing service provided by the NGS. More data can be found on the OPUS website.


Ephemeris - A table of the satellite positions at any given time and date. Ephemeris table can be found for both man-made satellites and natural satellites, such as planet and stars.

Link to comment
.... We all have the equiptment and technology right in front of us ....


That should be "limited" equipment and technology ....


About the best one could do with a handheld to get the "best" results with some confidence would be about 30 days logging ALL position solutions. 30 days will generally account for 99.9% of all errors/anomolies and give some confidence to the final result even though the final result would still be limited by precision issues.


Depending on the receiver make/model it is possible to record carrier phase from some recreational handhelds and post process this against the closest CORS site. Still has some limitations (accuracy wise) including precision.


Cheers, Kerry.

Link to comment

Thanks for everyones input.

I enjoy learning and have complex ideas sometimes that I need help in determining the validity of the thought.

I can see that this would entail time and detail that I am not ready to commit to,the readings were called in and adjusted from Houston,Texas,then we did the point conversion,The Surveyor running the Equptment knew a whole lot more than I could comprehend about the units and uses.


The accuracy that I require is not cm level, if I can get .001 which is approx. 5 feet + -,that means 2.5 ft. on each side of the mark,to me thats fairly accurate.

The accuracy of the Surveyors chains is no where near that.


By the by I was working at the base station of the RTK's and not the rover.


Thanks again Happy................................................................ Geotrails

Link to comment

Elcamino, Kewaneh & Shark and Kerry have done a good job of responding to

GEO*Trailblazer 1. If I'm not exactly sure what he means by a constant GPS

point other than some point where you could be assured the quality of a known

location to compare your hand-held receivers against. If this is the case, it already

exists in the more than 59,000 GPS stations published in the National Spatial

Reference System by NGS. When retrieving control points all you have to do is

select GPS under "Data Type Desired" and you will be provided just those points

where the position has been previously determined by GPS. The worse case in horizontal

positional accuracy of these points is approximately 10 cm, which is way beyond what

any hand-held unit is capable of. In addition, any conventional horizontal station of

1st, 2nd or 3rd-Order accuracy should be between 10 cm - 50 cm, which again is better

than any handheld unit. By the way, GeoTrailblazer 1, +/- 5 m means a total of 10 m,

not 2.5. Elcamino, I think GeoTrailblazer 1 is referring to .001 minute of arc. In the

Houston area .001 minute is 6.1 m in latitude and 5.3 m in longitude. For any of

you who are in the Houston area and would like to know more about the National

Spatial Reference System, I will be conducting an all-day workshop on this subject

for the Texas Society of Professional Surveyors. Their charge for non-CEU (continuing

education units) is $122.50. If you're interested, go to www.tsps.org, select "Education,"

then select "Take Your Pick Continuing Educations Seminars," then select "Development,

Implementation and Future of the National Spatial Reference System."

You can only register on-line.

Link to comment

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...