GPS Accuracy

[Gerald]

I have been using several Golf GPS Apps at my iPhone 3Gs and I kinda wonder how accurate GPS is...... I want to know if the accuracy is a few yards or more like 10 yds......

None of the systems will tell you anything about the accuracy, but the Freecaddie App I used the other day said, highest accuracy (19 yds)......

If the accuracy is 19 yds ...... I don't need a GPS ....... all I need is one of the markers 150 and 100 meter to center of the green, step out distances from there, do a little math, add or distract pin position and add or distract the windfactor.

I am very interested in the Garmin GPS S1 or S3, but it would only be a great tool if the accuracy would be about a few meters.....

If you are using a GPS in your car it means nothing if the thing says to turn left at 200 yds, if it is actually 175 yds, but if it says the center of the green is 175 yds and it is actually 160 yds or it is 190 yds ....... it is worth nothing !

How about the accuracy in Golf GPS Systems ?

[Stretch]

It depends.

If your device is capable of receiving an augmented GPS signal (known as WAAS in the US and EGNOS in Europe) and such a signal is available at your location, the tested position accuracy is roughly one meter (three feet) horizontally and 1.5 meters vertically, barring receiver error. These systems were developed for aviation navigation and assistance (for example, blind landings) and are certified for safety-of-life applications. Non-augmented GPS typically measures out to a position accuracy of around 2.5 meters (eight feet) horizontally and 5 meters vertically. Stated worst case for GPS is 10 meters, stated worst case for GPS WAAS/EGNOS is 7 meters.

[bplewis24]

Gerald, did you ever try GolfCard on your 3GS? I used it on my Android phone, and one of the features I appreciated most about it was it would have a signal strength indicator ranging from 2-10 yards or something. Most of the time it was accurate from 2 yards according to the strength meter. The first year I had it, I would often compare it against stand-alone GPS units used by other members of my playing group as well as course yardage markers. My phone was definitely very accurate. I believe it depends on the device though as it is hardware dependent. Brandon

[Fourputt]

Originally Posted by Gerald

I have been using several Golf GPS Apps at my iPhone 3Gs and I kinda wonder how accurate GPS is...... I want to know if the accuracy is a few yards or more like 10 yds......

None of the systems will tell you anything about the accuracy, but the Freecaddie App I used the other day said, highest accuracy (19 yds)......

If the accuracy is 19 yds ...... I don't need a GPS ....... all I need is one of the markers 150 and 100 meter to center of the green, step out distances from there, do a little math, add or distract pin position and add or distract the windfactor.

I am very interested in the Garmin GPS S1 or S3, but it would only be a great tool if the accuracy would be about a few meters.....

If you are using a GPS in your car it means nothing if the thing says to turn left at 200 yds, if it is actually 175 yds, but if it says the center of the green is 175 yds and it is actually 160 yds or it is 190 yds ....... it is worth nothing !

How about the accuracy in Golf GPS Systems ?

GPS accuracy is a percentage weighted number.  Most are advertised as being accurate to +/- 3 yards, but that only means that they have to hit that accuracy more than 50% of the time.  They CAN be off as much as +/- 5 to 10 yards when certain circumstances exist.  If for some reason reception is bad (heavy forest, steep hills close by - sometimes even heavy cloud cover can affect it), you can get erratic numbers.  If the receiver can only see 3 satellites at the time you need to get a distance it can be off because it is more accurate the more satellites it has to average out.  Also if the battery is low, the numbers can be all over the place (this happened to a buddy of mine - when he changed the batteries, it settled right back down).  You are also dependent on the accuracy of the person who took the original measurements and the programmer who applied them.

Considering all of the variables, it's a bit surprising that they are as accurate as they are.  That is why I carry a laser rangefinder and a GPS, and use both.  I have found inaccuracies with with the GPS, so I don't trust it unconditionally.

[Chief Broom]

I have a GolfBuddy GPS and to be perfectly honest it's pretty much useless.  Sometimes it can be very accurate and other times it's obviously way off.  That means I have to determine whenever or not I think the number it's giving me is to be trusted.  Pretty much useless.  I bought my GPS back when I sold my boat and I sure do wish now that I had spent that $400 on a good laser rangefinder.

[Gerald]

Originally Posted by Chief Broom

I have a GolfBuddy GPS and to be perfectly honest it's pretty much useless.  Sometimes it can be very accurate and other times it's obviously way off.  That means I have to determine whenever or not I think the number it's giving me is to be trusted.  Pretty much useless.  I bought my GPS back when I sold my boat and I sure do wish now that I had spent that $400 on a good laser rangefinder.

At last ..... a comment that is a reflection of my thoughts ...... "make sure I am not saying all other comments to be worth nothing", but I have serious doubts about the GPS systems, there are a bit too many variables, like the quality of the signal, the quality of the mapping, the mapping being up to date (like our course has been under the development of a big 5 year plan to upgrade the course and all maps I have seen including airials are from at least 3 yrs ago) ...... in total it sounds to me as investing $250-$500 on a gadget that is not reliable enough, that is also a bit of a hassle in the course and that may only be used in Europe if the committee makes a local rule allowing these things to be used in their tournaments.

It gives also a bit too much info, I would like to have only distances to the front, center and back of a green from the position of play, plus distances front and back of hazards, fairway bunkers, creeks, carry over trees, carry to a dogleg, etc.

My personal prefs for simplicity would go towards the Garmin Approach S1 watch, but it has a bit too less info.

For most holes played I (or we) don't need to know that the pin is 316 yds away, but an accurate reading within let's say 195 yds in can be very helpful or if you are within let's say 75-30 yds off when there are no markers in the course .... it would take some of the math out, based on actuall pinposition (which can be difficult sometimes as the pinposition can not always be seen very accurately).

So it seems to me that a yardage book plus a laser range finder is the best combination, which also gives you the option of adding details to your own yardage book, for the times only yardage books are allowed.

[ghalfaire]

The inherent accuracy of the GPS to know your location on the face of the earth is close to +/- 1 meter (probably better than that because they use a differential algorithm to calculate position using several satellites) .  But the operating accuracy might be compromised by the survey of the course or by the software employed by the machine.  It works by first finding your location then using the survey database to locate the "middle" of the green (or other target) and calculating the distance between these two points.  So if the survey is not accurate or if the software is lacking in some respect you'll get a bad reading.  But I use one (an old SkyCaddie) and have found it more accurate than the sprinkler heads for me (faster also).  But I will admit there are times when I question the GPS and do my own "observation and calculation" to verify (or not) its reading.  But not very many times or I wouldn't use it.  In practice and inside of 150 yards I believe it to be accurate to +/- 3 feet or so most of the time.  At distances further than 150 yards it is probably +/- 5 or 6 feet.

I should have added all the above assumes good signal reception from the satellites.

[bplewis24]

But I use one (an old SkyCaddie) and have found it more accurate than the sprinkler heads for me

I believe this bears repeating. Sometimes those things aren't accurate. It's rare, but at times the rangefinder has saved me from trusting an inaccurate yardage marker. Brandon

[newtogolf]

GPS is not as accurate as it could be thanks to our government.  Course changes are a totally separate issue, but even different hole placements can make a big enough difference in the information these devices report.  If you want the most accurate distances and not playing for handicap or tournament, use a range finder with slope detection.  If you're playing for handicap or tournament, go with a rangefinder without slope.  Once you get used to using one they can be pretty quick to get the distances with.

[NEOHMark]

GPS rangefinders are only as accurate as the folks programming the course markers on the other end of the equation (I believe it's usually done via some web-based satellite mapper like Google Maps/Earth).  Most of the time, I've found my GolfCard app to be very accurate, but there are a couple specific spots on a local course I've found to be inaccurate by about ten yds.  I still carry a laser rangefinder in the bag, but hardly ever use it anymore because GolfCard on my Droid really has proven to be 99% reliable.  Considering the tens of thousands of courses mapped out by the dozens of app writers, they've done a remarkably good job IMHO.

[Fourputt]

Originally Posted by ghalfaire

But I use one (an old SkyCaddie) and have found it more accurate than the sprinkler heads for me (faster also).  But I will admit there are times when I question the GPS and do my own "observation and calculation" to verify (or not) its reading.  But not very many times or I wouldn't use it.  In practice and inside of 150 yards I believe it to be accurate to +/- 3 feet or so most of the time.  At distances further than 150 yards it is probably +/- 5 or 6 feet.

I should have added all the above assumes good signal reception from the satellites.

This makes no sense at all since GPS accuracy has nothing whatsoever to do with distance from the target.  All it does is spot your location, then compare that to the information in its data base to come up with a number representing how far you are from a given target.  The accuracy involved does not change because of distance.

[retexan599]

I recently compared my Golfshot GPS readings with the 150 yard markers in the fairways at my club.  Some of the comparisons were different by as much as 5-7 yards, although most were within +/- 1-2 yards.  Also saw that the teebox distances were off quite a bit from the scorecard on several holes.  I submitted the current course scorecard to Golfshot (service available via their web site).  After they notified me that they had updated the data in their course database, the teebox distances are now correct and the 150 yard distances that were off now show as +/- 1 yard.  I did not retest on every hole, but my sense is that submitting a current course scorecard may trigger a reset of the entire map; not sure of that, but the data now looks better.  I plan to test some more when I get a chance. So, sending in a current scorecard may lead to more useful data.

[Fourputt]

Tee to green distances can't be compared between scorecard and GPS because scorecard distances are figured down the center of the hole around doglegs.  GPS is only capable of reading in a straight line.

[ghalfaire]

Originally Posted by Fourputt

This makes no sense at all since GPS accuracy has nothing whatsoever to do with distance from the target.  All it does is spot your location, then compare that to the information in its data base to come up with a number representing how far you are from a given target.  The accuracy involved does not change because of distance.

I believe you're correct in that there is no "step function" change in the error as a result of distance.  I don't know a lot about the golf GPS devices other than as a user but have worked with some other types of GPS systems.  I would believe that the distance to the center of the green would be the result of first calculation of the vector from where the golfer is to where the data base says the center of the green is and the second step would be to calculate the magnitude of the vector.   Finding the magnitude (distance to the center of the green)  of a vector is a process of square root of the sum of squares calculation.  Since the machine bit limits and multiplying and square root algorithms for this normally have an error tolerance given in percentage it would seem to make sense that the magnitude of the error would increase as distance increases.  But maybe not as the statement on my earlier post was a causal observation and not a studied result.  I probably should have said that to make it clear that I don't really know for sure precisely what the error is.   Just casual comparisons with my laser carrying buddies would seem indicate that the error is somewhat larger at larger distances to the green with the GPS.  But as someone posted earlier you're not going to find out for sure just how accurate the system is from the manufacture nor are you going to get access to the processing algorithms. So I guess we will never know.  In any case I like my old SG3 and find it helps my game and is, IMHO, better than the sprinkler heads and other yardage markers on the course.

[Fourputt]

In a simplified form, all a golf GPS does is receive the signal from whatever satellites are in its range of view at a given time.  From that information it is able to determine where the receiving unit is, based on a coordinate system.  All programmed points in it's data base are also assigned specific coordinates, and will simply use those coordinates in a simple mathematical formula to tell you the distance from the receiving unit to a given target.  It matters not whether the target is 500 yards or 50 yards as far as accuracy is concerned, because the receiver is in a fixed location as far as the calculation is concerned, as are the targets programmed into the database.

Vector is also irrelevant.  A golf GPS can't tell you direction, either by compass or the direction to the target.  Either function would take it into the realm of illegality under the Rules of Golf.  The only information you are allowed to get from it is distance.

[ghalfaire]

Originally Posted by Fourputt

In a simplified form, all a golf GPS does is receive the signal from whatever satellites are in its range of view at a given time.  From that information it is able to determine where the receiving unit is, based on a coordinate system.  All programmed points in it's data base are also assigned specific coordinates, and will simply use those coordinates in a simple mathematical formula to tell you the distance from the receiving unit to a given target.  It matters not whether the target is 500 yards or 50 yards as far as accuracy is concerned, because the receiver is in a fixed location as far as the calculation is concerned, as are the targets programmed into the database.

Vector is also irrelevant.  A golf GPS can't tell you direction, either by compass or the direction to the target.  Either function would take it into the realm of illegality under the Rules of Golf.  The only information you are allowed to get from it is distance.

I think what the GPS does, as you state, it to locate itself on the face of the earth and then look up your target point from a database and calculate the distance between those two point.  So in  in a simple explanation the only errors are in the location of the two points and those errors are independent of the distance between them.  However the two points have to be described in some manner for the computation of distance to occur.  This requires a coordinate system that is likely latitude, longitude and elevation although there are other types of coordinate systems to describe points on the earth.  The only way I know to determine the distance between two given points in 3 dimensions (or any dimensions greater than one in fact) is to calculate the vector between the two points and then calculate the magnitude of that vector.  It is this calculation that can (does) introduce more errors and I submit those errors are dependent of how far apart the two points are.  But likely this is small in comparison to the initial errors in location of the two point in the first place.  So unless there is something about this I don't know you need the vector even if the directional part of it isn't important to the golfer or displayed on the machine.  There is nothing illegal about the machine having the vector for the calculation as long as it is only capable of displaying distance and not other information the USGA says you can't have.

[Fourputt]

All that's required to calculate the distance is the Pythagorean Theorum.  No angular vector is needed to figure the distance between 2 points on a grid when the size of the grid is known.  Sides 'a' (GPS receiver location) and 'b' (data base target) are knowns, so figuring the hypotenuse is simple, and still leaves accuracy independent of the magnitude of separation.

[NCGolfer]

I have Golfshot for the iPhone and it seemed good at first but my confidence in it changed to the point where I now use a Bushnell Rangefinder.  I used the GPS side-by-side and it was correct maybe 40-50% of the time (within a yard or so) but other times it was off by more than 5-7 yards which is completely unacceptable to me.  And it seemed to vary course by course

For the $30 I put into the app, it was worth it but I feel much better having the rangefinder.