GPS for location of start/finish?

I think GPS coordinates are a very poor method for precisely locating points. The measurer's GPS has to be working properly. Then, The guy who needs to locate the point also has to have a GPS which also works properly.

There's nothing wrong with using longitude and latitude, so long as the party who has to use the coordinates is able to do so. Few are able to use GPS coordinates to the accuracy we require.

A measured distance from a nearby landmark is understandable and usable by all.

As for the accuracy of 42 degrees 56.505 minutes, that isn't very precise. As I figure it the earth is 25000 miles around at the equator, or 132000000 feet. This is divided by 360 degrees, or 21600 minutes.

We get 6111 feet per minute, or 6.1 feet for a thousandth of a minute. The coordinates used by Scott will, if accurately followed, put one within 6 feet of the proper point. One more decimal place in the minutes is needed to get within a foot of the right spot. Two more (5 beyond the decimal place) would be good enough to put the point within inches.

Pete,

I may be wrong here, but the point where Scott got the reading doesn't have a circumference of 25,000 miles. Say it's 20,000 miles! Wouldn't this make the readings closer to 4.9 ft for a thousandth of a minute.

One degree at the equator is equal to 1/360 of the earth’s circumference.

Latitude (north/south) degrees are all 1/360 of the circumference.

Longitude (east-west) degrees are dependent on the latitude. At the equator an east-west degree is 1/360 of the 25000 mile circumference. As one moves north or south each degree’s linear equivalent becomes smaller, until at the north or south pole each degree becomes infinitely small as a linear measurement.

Thus the uncertainty of the measurement is in the form of an ellipse, with the true value residing somewhere inside the ellipse.

If a GPS coordinate is used to describe a location, every effort should be made to establish a permanent mark, whether a spike with whiskers, a ¼" rebar, or something else. Then, when the GPS operator gets in the very-near vicinity, they should be able to locate that marker.

I know it is not always possible, but lacking a viable landmark, GPS coordinates in conjunction with other descriptions and placed-markers sometimes have to do.

It should be rare that a Start or Finish has to be so remote that there are no landmarks to reference. Split locations can be remote very often, and I have used the combination of vague descriptors and GPS coordinates more than I would like. But, if a Start or Finish is in a gathering place, isn't there normally something there to use as a reference?

Just my thoughts.

When I first embarked on my quest to establish the true length of the IHSA State Cross-Country final course in Peoria, I thought I'd do myself a favor by taking GPS readings of the various monuments placed around the course. I was passing through Peoria in November, a couple weeks after the state meet, when the marks were visible because the park had been mowed and the grass had gone dormant for the winter. I took GPS readings of the three points that define the start, the two that define the finish, and the five for every half mile along the course, all marked by metal discs about 10 cm in diameter.

I arrived back at the park in July, and pulled out my GPS and my list of readings. I couldn't find a single point. The grass had grown over them, and my GPS wasn't accurate enough to hone in on them. We were fortunate that another member of the party had brought a metal detector, or we might not have found them without a serious amount of extra effort.

My takeaway from that experience is that GPS data, at least as generated from devices people are likely to have at their disposal, is not accurate enough to mark points that define the course. It may have uses (calculating separation comes immediately to mind) but it is no substitute for permanent marks measured to landmarks.

Here is an example of just how much scatter one can get for the GPS readings of a fixed point at different times. My Etrex-H was set to display British Grid Refrences in metres.



Combining these data with another set taken at the other end end of the calibration course to give a total of 38 measurements I deduced that there was a 95% chance that the readings will lie within a circle of 6 m radius.

Of course, if I as a measurer had taken just a single GPS reading to provide a reference point and then later someone with a GPS tried to locate the point, they could end up even more than 6 m off.

For my calibration course there is a good view of the sky. When there are very substantial obstructions of the horizon the errors can increase many times.

When handheld GPS units first started to gain popularity I tried a couple of them. I had hoped GPS would provide a quick & easy method for locating points and for elevations.
I found very soon that GPS was pretty good at getting to within about 15 to 25 feet of a location but that the system was constantly adjusting and a single coordinate moved all around my front yard over the period of a day. The same experience held true for elevations. Forget any kind of accuracy in wooded areas or in urban environments. Trees and tall buildings just confuse the unit to the point where any data is useless.
In checking around I did find that the cost for a land based positioning monitor was several thousand dollars but the repeatable accuracy was similar to using a steel tape.
I still have my handheld and use it from time to time but it really can't be trusted

My fence post test. I sat my 305 forerunner on a fence post. Every ten minutes or so I marked the location. With several reading I had all but two identical. The disagreement on the two was 0.001 minute. I then ran another test. I marked the fencepost location and carried my 305 to my back deck 75 meters away. After a ten minute wait I put the 305 back on the fence post, waited a minute and marked the location. It took a circle of 8 meter diameter to enclose the several reading. I live in the very flat Rio Grande Valley about 45 mile from a Gulf of Mexico and the fence post was about 40 feet above sea level. The forerunner usually picks up 10 sat.

Bill Grass