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 Defining the endpoints of a cal course

 posted 10 November 2010 20:29
A measurer sent me his cal course map with the endpoints defined by straight line distances from 2 fixed objects on the side of the road. For example he measured from his PK nail to a lamppost just south of the nail, and to a lamppost just north of the nail. He didn't measure the distance along the direction of the road, but straight to the lamppost. A person trying to locate the nail (or where the nail used to be) would need to find the intersection of the arcs.

When there are no landmarks actually on the road, this seems to me like a much more precise way of defining the endpoint location than giving the distance in a direction along the road.

 Posts: 956 | Location: Rochester, MI | Registered: 13 April 2006 IP

 posted 10 November 2010 20:51 Hide Post
Mark,

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

 Posts: 879 | Registered: 26 October 2004 IP

 posted 13 November 2010 22:05 Hide Post
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 Wight

 Posts: 208 | Location: Hoffman Estates, Illinois, USA | Registered: 25 October 2004 IP

 posted 14 November 2010 08:54 Hide Post
Jay,

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

 Posts: 879 | Registered: 26 October 2004 IP

 posted 14 November 2010 11:19 Hide Post
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.

 Posts: 956 | Location: Rochester, MI | Registered: 13 April 2006 IP

 posted 15 November 2010 08:16 Hide Post
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

 Posts: 177 | Location: Brookline, MA | Registered: 24 October 2004 IP

 posted 15 November 2010 23:12 Hide Post
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.

 Posts: 723 | Location: Denver, Colorado | Registered: 09 May 2006 IP