I’ve been uploading my GPS tracks (GPX files off Garmin) to Routegadget for a while now, and the accuracy of route vs map / controls varies wildly.
Some days, you upload the route and everything fits almost 100% - good news
Other days, you upload the route and one side or the other (or multiple bits) are way off. So if you drag the route one way to fit with control 5, control 16 will be out (or vice versa.)
Am I right in assuming that the GPS route is always correct (as it seems to be for my non orienteering runs), which therefore means that if it doesn’t fit, the map is wrong / distorted? Which would help explain some of my mistakes...
Routegadget GPS accuracy
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Re: Routegadget GPS accuracy
Don't know much about GPS, but most non-georeferenced maps are massively distorted on a large scale (and usually accurate on a small scale). According to the Darnaway map, before the WOC2015 remap, the river Findhorn flowed uphill...
WOC2024 Edinburgh
Test races at SprintScotland (Alloa/Falkirk) and Euromeeting (near Stirling).
Test races at SprintScotland (Alloa/Falkirk) and Euromeeting (near Stirling).
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graeme - god
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Re: Routegadget GPS accuracy
On rare occasions (and with a GPS I don't use any more) I've had routes that start off completely wrong - in one case several miles from where I was! - although they tend to converge on the truth in the end. GPSs can also be affected by buildings, power lines, trees etc, so in an urban area I wouldn't trust them at all for drawing a route, forest areas maybe, and in sand dunes they're usually spot-on. You could try loading the route into Google Earth and see if it seems to match that, if the area's suitable.
- roadrunner
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Re: Routegadget GPS accuracy
How can something be very distorted on a large scale but accurate on a small scale? Surely it’s equally distorted as a %, except that the larger the scale the more you notice it. Or are you saying there are pockets of map accuracy interspersed with some really awful bits?
- Arnold
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Re: Routegadget GPS accuracy
Arnold wrote:How can something be very distorted on a large scale but accurate on a small scale?
Easily. We only really need local accuracy, e.g taking a bearing from an attack point. If the long path run to the next block is 2-3% too long/short, or at slightly the wrong angle, most people won't even notice.
Many maps date back up to 50 years, with perhaps 10-15 major updates over that time. If one block is very feature rich, but the adjacent ones aren't, the mapper may have pushed the block boundaries out slightly in order to fit everything in. At the next update, if things have changed, the distortion may be in a different direction. Over time it can build up significantly.
We are now trying to geo-reference our maps as they get updated. In some cases they are distorted so much that it is easier to do a complete redraw from scratch, so perhaps only gets done for A/B, not for C/D. Once achieved, map updating is easier, as a good GPS system will accurately locate every new feature. But even then, in a feature rich area local distortions may be needed. Point features on the map typically have a theoretical footprint of 10-12m across. If in practice they are smaller, and you need to show a bunch of close-together features that are each only 3m wide, some of them may still have to be in the "wrong place" - so even if 100% accurate your GPS track may still show you have "missed".
- Snail
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Re: Routegadget GPS accuracy
Arnold wrote:Or are you saying there are pockets of map accuracy interspersed with some really awful bits?
I meant what snail said.
But its worth noting that OS maps are often "map accuracy interspersed with some really awful bits", with the really awful distortions stuffed into hard to survey places they dont expect anyone to go (i.e. good orienteering terrain)
WOC2024 Edinburgh
Test races at SprintScotland (Alloa/Falkirk) and Euromeeting (near Stirling).
Test races at SprintScotland (Alloa/Falkirk) and Euromeeting (near Stirling).
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graeme - god
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- Location: struggling with an pɹɐɔ ʇıɯǝ
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