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Discussion: termite mound bingo

in: Craney; Craney > 2014-04-20

Apr 23, 2014 11:39 AM # 
jennycas:
Still not quite sure what was the problem with that one, but it got a few people - seemed to be too far across the spur.
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Apr 23, 2014 11:51 AM # 
Craney:
Yeah that fits with my view that it was too far from the bottom of the gully. I just don't see the point in that sort of control - even with the best technique you can't be sure you'll hit it and if you don't you get very heavily penalised because there's nowhere to relocate.
Apr 23, 2014 12:06 PM # 
Uncle JiM:
I agree, played it safe and came in from the track junction, was basically on target, but when it didn't appear, I turned left, still no control, so headed back and away from the gully.
Apr 23, 2014 12:48 PM # 
lazydave:
my biggest loss for the day was there but I just executed a great parallel error.
Apr 23, 2014 9:58 PM # 
Shep:
The most annoying thing is we were playing bingo while just over the road they were playing hungry hungry hippos.
Apr 23, 2014 11:01 PM # 
Nails:
major boo alert
Apr 24, 2014 12:45 AM # 
blairtrewin:
Murray's track (which I assume is from a GPS) indicates that the control is in more or less the right place, so it's presumably the surrounding detail which is a bit suspect.
Apr 25, 2014 1:35 PM # 
robplow:
Can we see Murray's track somewhere?

I wouldn't necessarily use someones GPS track as evidence of accuracy of a map or control placement - gps is not always that accurate. And many apps for GPS allow you to drag known points to match the map - maybe that is why they match - because he dragged the route to match the control site on the map. I don't know of course - just a theory.

The only gps route i have seen is Keely's (http://doma.orienteering.asn.au/show_map.php?user=...) which goes nowhere near the control (we're talking about 14 right?) But then that too may be due to poor correspondence of gps route with the map - if you assume that dogleg to the SW of 13 is not an error but actually where he slowed (hence the red) to punch the control then it moves the track close to the control (according to the map) at 14. Also for 15 the little patch of red (which presumably is him punching) is SW of the actual control.

The point is, a single gps track doesn't necessarily prove much. If you had a lot of tracks all showing a similar thing then maybe ... a good statistician would understand that.
Apr 25, 2014 1:53 PM # 
robplow:
I just found Murray's route on DOMA (http://doma.orienteering.asn.au/show_map.php?user=...)

You would want to check with Murray before coming to the conclusion you reached Blair. If we assume that the gps track is accurate - my first guess would be that the control is actually to the east of where it is shown - where his route turns 90 deg. And that would also help explain why his direction to 15 is so far off.

And in the light of this - looking again at Keelys track he also makes a sharp turn to the east of 14 (after applying the 'correction' I suggested above) and appears to be heading in the 'wrong' direction to 15 at first.

Then again other Doma maps (Dave Meyer, Krystal Neumann) appear to show 14 in exactly the right place.
Apr 27, 2014 2:26 AM # 
Robin:
Where the GPS route is on the map depends on what you did to try and match the route over the map image i.e which controls to which points on your route (as Rob Plowrigth has said) . I matched some controls on each day to a scanned map - to try and match all distorted the map a lot. With those I matched, others are then are off the actual route as is some of the route itself - what I saw on the round does not matche where my route is on the map. These maps have some relatiivaty issues I think and without a spatially rectified map we will not know how far things were out (but some definitely were). So not surprising that some people's maps and GPS route may show a control in apparently the right place and others do not. Also in general my GPS routes on a spatially located map are usually quite accurate.
Apr 27, 2014 6:28 AM # 
TheInvisibleLog:
The maps aren't georeferenced so a quickroute GPS track is almost meaningless.
Apr 27, 2014 7:03 AM # 
Shep:
That's almost, but not quite, completely untrue.
Apr 27, 2014 11:01 AM # 
robplow:
Robin - did you scan a map and try to match it to an imported gps route in ocad using F9? If so I am not surprised if you ended up with major distortion of the map.

Just scanning a map can result in significant distortions - try scanning an A4 map and then open as a background in ocad - it won't match well - unless you have a much better scanner than me. (for mapping purposes, working with non-digital base maps, the way to minimise that potential distortion was to carefully draw a grid over the base map and scan in small segments using the grid to align the segments in ocad)

To match a gps route to a map you are better off using something like quickroute that 'distorts' the gps route to match the map (which remains unchanged).

Looking at Murray's route again he seems to have matched the route to the map quite well -when he is running on tracks the route follows them quite well (compare with Keely's which doesn't - but he has clearly used a photo not scan, even more distortion). In the case of control 14 I am guessing the the point of punching matches the map well at controls 13 and 15 (change of angle and/or reduced speed) so I would expect the track to be reasonably accurate around 14 as well, so I would be moderately confident that my analysis above is on the right track - would need to check with Murray to know what points he 'matched' to be sure.

That map was made from Chris Wilmott photogrammetry - not georeferenced - but apart from that I would not expect any 'relativity issues' (unless there was careless copying/scanning of the base map). There may be some discrepancy between the O map and a georeferenced map over the whole area but it would be consistent and small enough to be unnoticeable (even to a mapper) over short distances. That is not to say there may not be occasional errors like a poorly mapped termie for example. But that is just the sort of 'human error' all mappers make occasionally.

It would be easy to georeference the map - if you have digital data (or a careful scan of a paper copy) of the nsw topo map. There is a 'how to' video in ocad help that tells you what to do - takes 10 min. Assuming there is no major distortion in the existing map the result will be accurate enough for all O purposes.

For once I am not going to take you to task for exaggeration Shep.
Apr 27, 2014 11:22 AM # 
Shep:
:)
Apr 27, 2014 11:29 AM # 
blairtrewin:
For what it's worth, I think Murray probably made a mistake on 15 (I'd assumed he had from the track, probably because I made a near-identical mistake myself) - the splits suggest he dropped 20-25 seconds on that leg - and hence the change of direction NE of 15 may not indicate a control location.
Apr 27, 2014 11:43 AM # 
robplow:
I never said the change of direction NE of 15 is the punching point - I am saying his punching point exactly matches control 15 on the map - you can tell because the line goes yellow - red -yellow - indicating his slowing to punch. ( I was saying the change of direction and change of color E of 14 probably indicates the rough position of 14)

If we assume that 14 was actually east of where it is shown on the map then Murray's bearing to 15 was probably accurate - he was just starting from the wrong place.

The fact that you made the same 'mistake' may be further evidence of the inaccuracy of 14.

A misplaced control (or the feature it is on being misplaced) can affect you both before and after the control.
Apr 27, 2014 12:07 PM # 
blairtrewin:
That's a fair call. Hadn't seen the red section of the track under the termite mound.
Apr 27, 2014 12:12 PM # 
robplow:
And you had somewhat misread what I wrote in my original posts
Apr 28, 2014 12:34 AM # 
Uncle JiM:
Here is my take of were it is. As mentioned earlier, I came in from the track junction, was on target, but when not seeing anything in front, I wondered right towards Laurina (Who was coming in from my right) before turning left. Still no control, so stopped, and headed back up, and away from the gully, then saw it in the distance

My anchor points for the GPS track on QR's is the track Junction, and the gate of the 2nd last control, so that shows the next control was in the right place, and confirmed again as I crossed the track, exactly as I thought I did
Apr 28, 2014 1:06 AM # 
Craney:
That's roughly where I felt it was too... Miles away from the gully!

This discussion thread is closed.