Alan's Ramblings - gps tag

OziGeocacheUK

Tue, 11 Apr 2006 09:41:13 GMT

I've been tinkering around for a while wrapping the API to OziExplorer in Java. I've finally knocked together an application that uses it. integrates information from GeocacheUK.com and Geocaching.com into OziExplorer. OziExplorer is a GPS mapping application and GeocacheUK is a database of geocaches in the UK. Hopefully this post will help google steer anyone who is looking for such a thing towards it ;-)

Getting accurate height datums

Fri, 27 May 2005 02:15:18 GMT

My Garmin eTrex Summit has a built-in altimeter, and although it can auto-calibrate from the GPS fix, it isn't that accurate and it takes a while to do. The Ordnance Survey have been gradually removing the spot height and benchmark information from the 1:25,000 maps as mostly they aren't used for surveying any more. Both trig points and benchmarks need 'maintaining' by being resurveyed at intervals as they are subject to ground movement, so it is understandable that the OS isn't bothering if they don't use them any more. However they are still good enough for calibrating an altimeter which isn't accurate to less than a metre anyway, the problem is actually finding the information! I was browsing around the web looking for something else entirely and I found that The Planning Inspectorate provides a map facility that allows you to drill down on th the OS MasterMap maps which still have the height information on them. Go here and click on the "Select a Point" or "Select an Area" icons and keep drilling down to you see the individual property boundaries on the map. From that point on you will be able to see all the spot height and benchmark data, to the nearest centimetre, and there are a lot of them, so you should be able to find a convenient one to use for calibrating.

Yet another eTrex Summit

Thu, 26 May 2005 04:28:37 GMT

As I documented at length earlier, I bought a Garmin eTrex Summit in October of last year, and had no end of problems with it powering itself off for no apparent reason. Garmin replaced it (twice) and the third unit was OK-ish but still tended to turn itself off occasionally. After an email exchange with Garmin I said I'd try to track down exactly what the problem was. I persevered, and the clincher was when I turned on the inbuilt magnetic compass and the unit immediately turned off - a fresh set of batteries, no low battery warning, it just turned off. Once this had happened a couple of times I felt I had enough information to send a detailed report to Garmin. I'd expected to be told it was a fimware issue and that I'd have to wait for a fix, but I was pleasantly surprised to be told that they were just releasing a new version of the Summit and that they would send me one. I was even more surprised when it turned up a day later! It appears that I've been sent a pre-release unit, as it has features that aren't mentioned yet on the Garmin product page for the Summit. It turns out that Garmin have made a whole bunch of improvements:

The biggest change is the addition of support for WAAS (USA) / EGNOS (Europe). This is a satellite-based differential GPS system - basically a network of accurately surveyed ground stations watch the GPS signals, calculate the difference from where the signals says they are and where they really are, send the correction back up to some other satellites which then rebroadcast it so that WAAS/EGNOS-enabled GPS units can use the correction to improve their accuracy. At the moment the European system isn't fully deployed, but when it is it should give a 10x improvement in accuracy - indications are that the accuracy will be around 1 metre.
New 'Trip Computer' page which allows you to select 5 fields from a list of 31 to display. The list is very comprehensive - just about every parameter I can think of is covered.
When navigating to a waypoint the map page can display a line from where you are now to the destination (as per the older model) or a line from your starting point to the destination. This is the way the GPS12XL works, and it's much more useful as it allows you to easily see if you are wandering left or right from your course.
The compass can be turned off as well as on by holding down the PAGE button - on the older model you could only turn it on.
The track log can be turned off, and the point collection mode can be set to one of three modes:
- 'Auto' which has 5 different resolution settings.
- 'Distance' which allows you to specify the distance between sucessive points.
- 'Time' which allows you to specify the time between sucessive points.
More internal memory, which means the following changes:
- Waypoints per route up from 50 to 125.
- Track log points up from 3,000 to 10,000.
- Points on saved tracks up from 500 per track to 750.
The fairly useless Hunt/Fish page found on some of the other eTrex models is now available.

There are a few other minor changes - you can view pressure in hPascals, and they've modified the 'Units' setup screen so that if you select 'British Grid' as the position format you can only have 'Ord Survy GB' as the datum - a common mistake that people make is to set the position format but not select the correct datum, which causes all grid references to be several hundred metres out. The one thing they haven't fixed is that you still can't move an existing waypoint to your current position - this was a very useful feature on the GPS12XL - when EGNOS is deployed I expect to find that many of my waypoints will be inaccurate, and there is no easy way to correct them other than reentering them.

Overall Garmin have made an already impressive and useful GPS even better, and despite my best efforts I haven't managed to get the new one to power itself off, so it looks like they've fixed that problem too. Yay Garmin!

Useful directory of online maps for the UK

Sat, 07 May 2005 07:13:28 GMT

While I was looking for the format of links to the OS GetAMap service for my last post I came across this useful list of online maps for the UK, which it seems has been lifted straight from Wikipedia - why they didn't just link to the Wikipedia page I have no idea. I particularly liked the content of the Magic website, which as Wikipedia says "is a web-based interactive map intended to bring together information on key environmental schemes and designations in one place", although the presentation is a bit clunky when compared to something as slick as Google maps, although Google in turn suffers from not being based on the OS mapping, so it's pretty useless outside of urban areas.

And by the way, if you do want to link to somewhere on the OS's GetAMap service, the format of the URL is
http://getamap.ordnancesurvey.co.uk/getamap/frames.htm?mapAction=gaz&gazName=g&gazString=
with the normal OS grid reference appended onto it, e.g. http://getamap.ordnancesurvey.co.uk/getamap/frames.htm?mapAction=gaz&gazName=g&gazString=TQ302795

One problem solved, another remains

Fri, 22 Oct 2004 10:38:50 GMT

Well, my third eTrex arrived today, a nice new one and so far it seems fine - despite the abysmal weather I took a stroll to try it out. I'm still puzzling over exactly where the Bleaklow fence line goes between Shining Clough and Woodhead, so I parked up above Woodhead tunnel and walked up onto the Trans Pennine Trail (the old turnpike road) then wandered westwards towards Woodhead Bridge. That gave me a chance to reccie the other side of the valley and figure out where the fence goes. It appears to come down to the Longdendale trail along Smithy Clough, but I've still no clear idea of where it goes between Stable Clough and Shining Clough. One of the nice things with the Summit is you can use it for doing resections - because it has a built-in compass you can mark your current position, then take a bearing from where you are and project another waypoint along it. If you do this three times from different vantage points and join up the three resulting lines (dead easy in OziExplorer - just set up three two-waypoint routes and display them),you get the traditional 'cocked hat' that shows you where the point of interest is.

As I said in my original post, I'd also paid £20.00 to Geomantics for some cleaned-up SRTM height data for OziExplorer. Well, it eventually arrived - in a format that's completely incompatible with OziExplorer, despite the website clearly saying " We supply geo-edited SRTM data in a suitable format ... for use with 3rd party programs such as Bryce, Terragen, OziExplorer, 3D Studio, Photoshop etc." Bollocks. It's supplied as 5km squares of 16-bit raw height data, and OziExplorer doesn't understand that format at all. Eventually, after repeated requests for information, Geomantics told me I had to download a 3d terrain modelling application, and a plugin, and re-reference and convert all the data - and I have 74 files worth, although I don't want it all. You'd need to have quite an in-depth knowledge of GIS and earth coordinate systems to be able to do this, which leaves me out. Needless to say, I've emailed them and the electronic payment service they use to ask for my money back, it will be interesting to see if I get it. I've also suggested that they remove the frankly untrue claim that the data can be used with OziExplorer from their website - if you get here via google and are intending to buy their data for use with OziExplorer, take my advice and don't.