Defining the endpoints of a cal course

Mark,

I agree that two interesting arcs is best. However, The use of Longitude and Latitude readings as a guide could help.

And if you defined the point by fixed measurements from three fixed objects there would be no ambiguity whatsoever, as there would be only one point that met that description.

I'm also wondering how many decimal places one would have to go to get accurate enough to locate the ends of a calibration course with GPS coordinates. When I was doing the groundwork for the measurement of the cross country course in Peoria, I visited in November, a week or so after the state meet. The IHSA had set permanent metal markers in the ground at the ends and center of the starting line, both ends of the finish line, and the five half mile splits along the course, and after a thousand kids had run the course, and when the grass was dormant for the winter they were easy to find. So I took my GPS device, laid it on each marker, and recorded the readings. When we got back there in July, the grass was high and the GPS was accurate only within the 5-10 meters that it usually is. We needed a metal detector to find most of the IHSA's marks.

Based on that experience, the only thing I would use GPS coordinates for is split points that I couldn't measure to anything else. I certainly wouldn't use them to describe anything that requires the accuracy that the end of a calibration course does.

Jay,

I agree with you on the GPS points. It's a good reference, but that's about all.

Even when I provide other descriptions, I sometimes also list the GPS coordinates of mile marks. But I don't list GPS for the start, finish, or turn-arounds, because I'm afraid the race director will use those instead of my other descriptions of those point locations.

The discussion on GPS accuracy may be a bit of topic, but got me wondering about my personal GPS. I use an older Garmin GPSMAP 60CSx. Garmin advertises an accuracy of <5 meters, typical.

GPS accuracy:

* Position: <10 meters, typical
* Velocity: .05 meter/sec steady state

DGPS (WAAS) accuracy:

* Position: <5 meters, typical
* Velocity: .05 meter/sec steady state

If I had only the GPS coordinates for an end point, I may need to borrow Jay's metal detector.

Google Earth may have a somewhat better accuracy. It displays coordinates to 6 places to the right of the decimal point. (ie. 41.680370 -71.127366). That implies an accuracy of about 11 cm. (where 1 degree = 1 nautical mile)

Thank you. -- Justin

Mark,

I read your thoughts on not giving coordinates for endpoints and turnaround locations, and thought that was a valid concern. Almost got me to stop putting them on my maps.

But, I then remembered why I started putting them on my maps in the first place - to give an indication of accuracy. If I give good descriptions of Start and Finish locations, those will likely be found first. If (and I know this is a big assumption) the user compares the reading on their GPS and the coordinates I provide, they will likely see a digit or two difference. They should then figure out that the split marks won't be precisely where the coordinates show on their device, but they will have a decent indicator of how close they may be.

I always put the disclaimer on my maps that GPS coordinates are only to be used to get close to the marks, and are not to be used to re-set lost marks. Hopefully, this is enough of a statement that they should not expect the nails to be exactly where their GPS shows my provided coordinates.