GPS 'Accuracy' Experiment

[DaveLeeNC]

I am on the beach in SC for a few weeks. This gave me an opportunity to explore something that I had been curious about.

 

I started back in 2006'ish with a GPS device. I was not happy with the accuracy and its inherent inability to answer my most important question (how far is it to the pin from inside 100 yards). So I ended up a laser guy.

 

But the Bushnell Neo was such a great bargain that I bought one for 'supplemental use' (occasional 'blind layups, mostly). The accuracy of this device seemed better (I would even say fine for the use I stated). But it never seemed as good as some folks seemed to think that their own GPS is.

 

So I did an experiment this past 10 days. I went out in basically 4 directions to a distance of around 70 to 170 yards and set 4 points in my rangefinder. Then from a FIXED location with a pretty darn good view of the sky I would take measurements to those four points and record them. I would always wait at least 45 minutes between measurements. I did this 63 times (a total of 63 x 4 measurements taken).

 

Note that I had no idea what the 'right distances' were. I was just going to measure how consistently the device would be able to give the same answer (right or wrong) to the same question.

 

i would then average the answers for each of these points and assume that (for the purposes of this experiment) was 'the right distance'. This distributions were relatively symmetric. And I then calculated the errors.

 

The largest 'error' that I saw was 6.1 yards (away from average). The largest range that I saw was 117 to 127 yards. The smallest was 165 to 172 yards. The standard deviation (SD) of the entire set of data was 1.66 yards (somewhere between 2 and 3 SD's would be personal estimate of accuracy - call it 2.5 which is then 4.15 yards). Clearly any mapping inaccuracies are an adder on top of this.

 

FWIW. Note that I did this same experiment a good while back in my hometown of Pinehurst, NC. I had always wondered if somehow GPS accuracy there is a bit worse than normal (possibly due to WAAS considerations). The SD that I got there was 1.55 yards which is about the same. However, we do both use the same WAAS station, I presume (somewhere in northern Ga. from what I recall). So maybe still things are better elsewhere.

 

dave

[ironmikes]

thanks for taking the time to do this and reporting your findings. seems like if you map the course on your own the fixed points could be fairly accurate. I like you have always had doubt on the shots inside of 100 yards. have doen it a few times I hit one over the green and think " gps said it was 80 to the back of the green but it played more like 65 as I flew my lob over the back , no way I hit my lob that far hmmmmm ". have to admit using the laser I have my mind at ease as I know the distance to the flag and take slope ,wind, etc into account.

[DaveLeeNC]

[quote name='ironmikes' timestamp='1346598719' post='5574304']
thanks for taking the time to do this and reporting your findings. seems like if you map the course on your own the fixed points could be fairly accurate. I like you have always had doubt on the shots inside of 100 yards. have doen it a few times I hit one over the green and think " gps said it was 80 to the back of the green but it played more like 65 as I flew my lob over the back , no way I hit my lob that far hmmmmm ". have to admit using the laser I have my mind at ease as I know the distance to the flag and take slope ,wind, etc into account.
[/quote]

The problem with mapping your own course for accuracy is that this process has the same inaccuracy. So your resulting accuracy would inevitably be worse than my numbers (although better than a couple of mapping errors that I have encountered).

dave

[ironmikes]

[quote name='DaveLeeNC' timestamp='1346602371' post='5574395']
[quote name='ironmikes' timestamp='1346598719' post='5574304']
thanks for taking the time to do this and reporting your findings. seems like if you map the course on your own the fixed points could be fairly accurate. I like you have always had doubt on the shots inside of 100 yards. have doen it a few times I hit one over the green and think " gps said it was 80 to the back of the green but it played more like 65 as I flew my lob over the back , no way I hit my lob that far hmmmmm ". have to admit using the laser I have my mind at ease as I know the distance to the flag and take slope ,wind, etc into account.
[/quote]

The problem with mapping your own course for accuracy is that this process has the same inaccuracy. So your resulting accuracy would inevitably be worse than my numbers (although better than a couple of mapping errors that I have encountered).

dave
[/quote]


Dave, had to reread your original post again. seems like I had it backwards whew ! looks like my choice to stick with the laser makes even more sense to me now. thanks for clearing that up for me .

[jaskanski]

The question on everyone's lips, Dave, is do you have a level of ability to discern +/- 4 yards with your iron play? Are you really that accurate?
The beauty of a given yardage between two fixed points (eg an on course yardage marker post) is that they are seldom wrong. Even if you care to measure their accuracy with a GPS device or laser (as many people do) the yardage marker never lies.

[DaveLeeNC]

[quote name='jaskanski' timestamp='1346604910' post='5574495']
The question on everyone's lips, Dave, is do you have a level of ability to discern +/- 4 yards with your iron play? Are you really that accurate?
The beauty of a given yardage between two fixed points (eg an on course yardage marker post) is that they are seldom wrong. Even if you care to measure their accuracy with a GPS device or laser (as many people do) the yardage marker never lies.
[/quote]

4 yards is plenty good for most full iron shots (even though 4 yards is sometimes enough to influence how/what I swing). But the real accuracy is not 4 yards. It is at least 4 yards plus mapping errors plus a possible calculation bias that I was unable to assess (probably quite low). However as I stated I became a laser guy primarily due to my needs inside 100 yards. And most certainly 4 yards matters to me in those cases.

dave

[jaskanski]

[quote name='DaveLeeNC' timestamp='1346607109' post='5574593']
[quote name='jaskanski' timestamp='1346604910' post='5574495']
The question on everyone's lips, Dave, is do you have a level of ability to discern +/- 4 yards with your iron play? Are you really that accurate?
The beauty of a given yardage between two fixed points (eg an on course yardage marker post) is that they are seldom wrong. Even if you care to measure their accuracy with a GPS device or laser (as many people do) the yardage marker never lies.
[/quote]

4 yards is plenty good for most full iron shots (even though 4 yards is sometimes enough to influence how/what I swing). But the real accuracy is not 4 yards. It is at least 4 yards plus mapping errors plus a possible calculation bias that I was unable to assess (probably quite low). However as I stated I became a laser guy primarily due to my needs inside 100 yards. And most certainly 4 yards matters to me in those cases.

dave
[/quote]
I would say 4 yards matters to just about everybody inside 100 yards, but i would re-iterate my point about [i]your [/i]accuracy. Do you actually possess the ability to hit precisely hit 96 yards on request to reach a pin, regardless of it's relative position and the need to (a) release the ball, or (b) spin the ball back (c) wind direction and strength (d) trajectory (e) none of which can be calculated by the GPS??
If you do, then I'd say the accuracy of your GPS is the least of your worries. Conversely if you don't, then the GPS is hardly worthy of criticism.

[DaveLeeNC]

There are shots/circumstances in that distance range where, if I am more than 4 yards long/short, I am surprised/irritated. But of course with a GPS you don't know the true pin position even with perfect mapping (although some devices handle this better than others).

So the 4 yard discussion isn't relevant (other than as a lower bound for the inherent variability in my device).

dave

ps. I need to make a clarification here. One of the 'circumstances' that I refer to is a perfectly struck ball (it happens :-) under the right conditions.

[ironmikes]

Well for me Dave, 4 yards can be a big difference. I play a lot of places with elevated greens and 4 yards turns out to be a lot more than that . add any mapping errors and that adds to the wrong yardage. not getting the yardage to the pin correct I am miss clubbing, even with perfect contact. my big factor is knowing the yardage and making a confident swing knowing I have the right club. some of the places I frequent have a lof of water to carry but if you go long you are in trouble . I can live with a bad swing resulting in a bad shot but hitting a well struck iron only to see it in trouble due to a bad yardage , that can be avaoided.

[DaveLeeNC]

I could live with 4 yards.

But the truth is that I don't know what it is.

dave

[Tjalfe]

If you can get a hold of the raw NMEA data from the gps, which is available in standard 9600baud UART format on, at least the newer skygolf devices, you can download ucenter from ublox.com which will plot the deviation map for you. with the higher end ublox GPS engines, their high precision engine kicks in around 10 minutes after cold boot, which gives less than 1 meter deviation