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Control Work

GEO*Trailblazer 1
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I was just wondering how many of us have gone into checking out the Control for GPS guidance.


That is having control points you frequent to see how close your readings are each day.


I have been doing it for about 10 years and have slowly improved my math and methods.

I just had to find out for myself,how close you can get on a "Mean average".


When selective availability was turned on I used to have to calculate each day the random error inherent in the system,which has led me to this discussion.


My First Mapping project.



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I use the tracking screen of the GPSr. On the 120 ft scale, I can see how wide my N-S tracks and E-W tracks are over the same routes for 20-30 times. Surpisingly, most tracks are +/- 50 feet, but I find the locations of highways and interstates not tracking correctly. I blame that on the GPSr programming that whoever drew the map did not have accurate location.

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Most all GPS download ephemeris data from the satellites which is corrected by the USAF at Schriver AFB near Colorado Springs Colorado. They will issue notices if there are Satellites are off the air or outages will affect accuracy but this is rare, maybe a few times a year. We can overcome the outages if we have time to wait and it is usually not enough to matter for most purposes. It may matter if you are trying to do something very accurate at say 600 MPH if you know what I mean... This ephemeris data will tell the GPS where all the birds are which are and are not available in the constellation, What Coordinated Universal time is, all the synchronizing timing etc, all so the radio triangulation can happen...


All timing between the birds is atomically accurate with Atomic clocks on each bird and synchronized from the ground. For dual channel work we can pull more than one signal from the birds and with DGPS from the US Coast Guard in many locations nationally we can correct for Ionospheric distortions, and we can also set up a DGPS Local network for local sitework. With Real Time Kinematic (RTK) GPS we can set up and let it run for hours to get a fix from many birds and this can accurize things even more if need be. But we can see right from the computer on the GPS how accurate we are and if that level of accuracy is acceptable to us. If we want more accuracy, we simply wait longer, if it is that important to do so. In Geodetic work it would be. Setting a construction staking point for a concrete floor would not. So for the most part it is not something we really even think about anymore. The Pro gear will tell us if we are good to go or not.


On consumer grade stuff, the accuracy resolution is really only +/- 5 feet anyway. Close enough for horseshoes and hand grenades. Using it, I figure I could hit my car with a horseshoe if I needed too. :-) As for consumer grade GPS mapping, well, I am not even sure what datum they really use, but that would surely explain all the little tracking errors. I Run NAD 83 in my GPS and I see tracking errors in the mapping all the time. Not bad but who knows... I have WAAS correction then I don't, then I lose track on a bird or 2, and the accuracy goes up and down, all the while I am on one datum, and the onboard map is of another, except for when it is not. See? It is all relative in the consumer end. For the most part they work well enough and we are not lost. All that is due to the lack of accuracy that is not built into the consumer GPS in the first place, not a matter of GPS constellation accuracy. It is still close enough to get you to a Hiding Triangulation Station!


Overall Geo, there is more to all this than I have explained but anyone can easily study up if they like. This is really more of a GPS topic than a Bench mark Hunting topic anyway. These days, it is something the USAF and USCG concern themselves with more than anyone else does anymore. We are post Selective Availability now. The lacking accuracy if any would be too small for most users to ever realize, and in the cases we can reveal it, Most all GPS units will tell you the kind of accuracy they are able to render at any given time. For Pro gear, a reading taken over time will get us where we want to go either way.



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Thanks,great explanation.


The reason I asked is I have a new program which allows you to calibrate Images of almost any type to coordinates to five decimals in the WGS 84 format and 11 decimals in the other.

I have been working with the new satts images,and it is awesome.the calibration of the imagery is the hardest for me,but have figured it out close enough for me.


Moving,yes the GPS has more error than the aquireing of control points.

you are right the longer you wait the better the average(MEAN).

I have a P.O.B. I use each time I start on a track....it is always less than 5' when I start,and upon return to the same point is less than 5'.


It has been really interesting going from not having any info and pretty much figuring it out in my head and as I have become more advanced,obtained better equiptment and payed more attention to detail, so have the programs(or the abilities to find them).


I am a disabled measurement tech and enjoy calibrating instruments,that was my job before I got poisened.

So this helps me to keep my mind active in things that most think frustrasting,math.


I will add the images I posted were done pre SA.......before civilian had the goodies to connect to a PC and I did not have a computer yet either,topozone was just getting started as well as geocaching.

Those maps are over 10 years old now.


I could figure alot of these things out faster if I was the old me but have to ask more ?? cause I caint member sometimes.


These forums have been a great help,as well as keeping me healthy mentally and physically,I sure need it,but just can not get out there and do it like I use to,and want to.


Good thing I have a wife that tolerates and likes benchmarks.

Gota get her to drive alot.well 1/3 of the time anyway.

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Hi Geo,


Well, If it well help you to know, these days since SA was turned off, you really could skip the step of going to your P.O.B. and calibrating as well as comparing. That 5 feet you are seeing is pretty standard for consumer grade accuracy. it really is something you should feel you can count on. Besides, these things are pretty self calibrating now as far as the network is concerned, and so is the end user gear. This could save you time, if you like.


Moving at most any speed, the satellite will track up quite nice and exact, but you need more than four birds to do it well. It is important to note that our government has some rather large cylindrical rocket powered objects that count on this as well as some other things that fly high and faast. Did I say faaast? well I meant faaaaaaast. So, no problem there. We are not trying to acquire the location of a Millimeter at that speed anyway.


You are doing fine Geo. Happy Bench Mark Hunting.



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OK.....I know this will cause a stir and some arguements, but I stand by my statements:


I have used three different Magellan units over the last 10 years and each gets better. I currently use a SporTrak TOPO with WAAS capability. Of the three P.O.B.'s I use almost daily, I am well within the 6 foot range. My calcs show generally at 3 feet with the WAAS engaged. This is better than the manufacturer claims.


I am also able to recieve TWO of the WAAS sattelites even though Magellan says it is doubtful to be able to do so.


(Besides the Eastern U.S. one, I am either receiving the African one or the South American one on accasion, probably the African one given that I am in Maine. Due to sun streaming, I doubt I am recieveing the Wetsrn U.S. sat.)


My claim is that these units, IN GENERAL, are probably getting more accurate than the manufacturer is willing to lay claim to (lawsuit time!) and are probably better than we, the general public, need.

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P.O.B. = Point of Begining


A reference point at a known and consistent location. I use a BM site in my backyard laid out by surveyors 60 years ago, one in the parking lot at work laid out by the DEP 10 years ago,13 miles from home and a Hilltop location halfway between the two whose position has been estimated by myself.


BTW.......I also claim a 3-6 foot accuracy in elevation with my GPS as compared to known elevations (BM's), Topo maps and an altimeter mounted in my Jeep.


EDIT NOTE.....corrected spelling.

Edited by Spoo
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You are correct, the consumer grade technology is becoming more accurate at the consumer level price point. This is a very good thing! But as I said, It is because of the technology you hold in your hand, not that of the network. The network with the right gear can get you to the single millimeter. As Geo pointed out in his original post, before SA was turned off, if you had any accuracy at all you had to verify it up front at a known POB and a sync up with DGPS. You could just acquire DGPS on the fly and all but to be sure, many chose to use a known start point. So no argument there! :-)


WAAS is nice when you can acquire it. Sometimes you get more than one signal, other times none. It will only accurize the Consumer grade so much. With only what you have, you only have so much. But it is enough. I don't see a lawsuit problem though, because it is not the manufacturer that certifies the measurement taken, it is the end user. Further the only people who are legally certify a land measurement are Licensed Professionals such as Surveyors and Engineers. That is a critical distinction. The person holding the license is the only one who can certify anything which will hold up legally. It is a very definite criteria. The Onus is on the professional. The equipment or technology of any kind is simply the professionals tool of choice.


For those who may want to observe this closely, project a few waypoints to local NGS triangulation stations. Use NAD 83 Datum and the Degree Minute and Second format (dddmm.sssss). Also, remember to use the real NGS datasheet. Make sure the data you use is not scaled. Try to enter them to the nearest resolution you can. Most will resolve to two places to the right of the decimal in a consumer grade GPS, though the datasheet will resolve to five places or more. you should round up if 5 and over, down otherwise. This will be close enough. The nice thing is that this way you will have places to go in the field wherever you are should you feel the need to check your accuracy. Either way, the unit should be fine if it is working properly and there are lots of people working 24/7 to make sure the network holds up their end. For the most part, and particularly with the consumer, single channel units with or without WAAS correction, testing against a POB is not all that necessary anymore.

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My Garmin GPSMAP 76S has resolution choices of 0.001 minute (about 6.1 ft for latitude) or 0.1 second (10.1 ft for latitude) and I wish it had a way to display one more questionable digit.


With totally open sky, WAAS on, and a good satellite constellation, I can stand on a waypoint and get distance readings that are within 3 feet most of the time. I conclude that averaging would get me a better Lat Long value than the unit will display.


To squeeze the most resolution out, I sometimes move off the position I'm trying to measure until the display flips back and forth between two digits, and call that the 0.0005 minute point, and then adjust for the distance I've moved. It's a lot more work than having another digit in the display.

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