Need to record a series of locations in lat/long

[Wombat]

I've just found out that part of my upcoming trip to Sierra Leone will involve surveying artifacts/buildings and recording their location.  I am "spent out" financially on this project, so I'll need to use tools I already have.  I was trying to avoid carrying yet more batteries (other than those needed for 2 cameras, 1 gopro, a flashgun, and needing to recharge 2 mobiles) and I will mostly be where there is not reliable mains electricity (i.e. Sierra Leone, as there is no such thing as mains electricity there).

So, all I have is an ancient Garmin Etrex I haven't used in years, and my Android smartphone. The Garmin is of an age to be a "serial only, connection" one, and I do not have access to a PC with a serial port.  So, I'm wondering if I can merely switch the Garmin on as we arrive at each location, and then note down the lat and long after a bit, and turn it off again to save the batteries.  Otherwise I could use my phone, but is there an app which allows me to read lat and long, rather than see my location on a map, which is little use, as the map will almost certainly be blank in that area.  OSMaps is not bad in SL, but out in the wilds, the maps are somewhat devoid of features.  I'll need to survive doing this for a week.  The total trip is 2 weeks, but a fair bit of the work is not relevant to GPS functions, as I'll be in a shed in Freetown!

Do I take several packs of eneloops and the Garmin, or am I better off with the power pack and my Sony Xperia Z?  The phone will be pretty useless in SL, as to talk to people I'll need a local SIM in a cheapo basic handset.

If I take the Garmin I'm hoping that some sort of USB/Serial port adaptor is available to download tracks/locations onto something on the PC, but writing down the lat and long together with the building survey notes would be fine.

I need to keep the weight of my luggage down, its already going to be a problem, carrying somuch power storage with me.  When in Freetown, hopefully my hotel will have 240v form its onsite genny relaibly enough I can top up camera batteries and the power pack.

[frankly frankie]

So, I'm wondering if I can merely switch the Garmin on as we arrive at each location, and then note down the lat and long after a bit, and turn it off again to save the batteries.  ... [2 week trip] ...
Do I take several packs of eneloops and the Garmin, or am I better off with the power pack and my Sony Xperia Z?
...
I need to keep the weight of my luggage down

If you're going to transcribe the lats/longs you might be as well off with the phone I think.  There would be loads of apps that will display lat/long - one that I like is called 'GPS Essentials' - however one issue might be the precise format displayed - eg dd.mm.ss.ssss or dd.mm.mmmmmmm or whatever - it's a pain if you have to convert between them - the Garmin has the advantage here as it is easy to set up the precise display format you need.

Switch  on, note the location, swicth off, repeat - I don't much like the sound of that, because you will have to wait quite a long time after each switch-on to be sure you are getting a reasonably accurate fix.  The phone presumably you would be happy to leave that on - though GPS use does shorten battery runtime.
The older Garmins generally have a 'battery save' option** which I think would be better than switching off and on, in conjunction with lithium AAs which give a longer runtime in a Garmin than even the best NiMHs such as Eneloops.  I would take 2 or 3 sealed blister 4-packs of Energizer lithiums. No charger required.  Flying should be OK as long as they are unopened original packaging.  And with the battery save mode enabled, I would have the Garmin on all the time as long as you are outdoors.  Very old Garmins are more battery-hungry than the newer models, but even so 3x4-packs should easily last long enough.
NB on the last point - lithiums are half the weight of Eneloops.

As well as the position, set up a data field on screen to display 'GPS Accuracy' - so that you can be fairly sure you are getting a good fix.  You can do this on a Garmin and with the GPS Essentials app I think.

** Battery save is in the setup 'system' menu somewhere - what it does is the GPS polls the sats less frequently - typically once per 5 secs instead of once per second - wouldn't be an issue for your purposes in fact I never found it a problem even when following a route on a bike.  Battery runtime is extended by about 40%.  Shame the newer Garmins have dropped this feature.

[mcshroom]

USB to etrex cables definitely exist. I may even have a spare in a box somewhere from my old Geko.

[frankly frankie]

If you know you can get the data off the Garmin, then that is definitely favourite - because you just click a button to record a waypoint at location.  Though there may will be a limit to how many.

[fuaran]

Yes, you could use the Etrex.
USB to serial adapters are available cheaply, ie £3 off Ebay. You would also need an Etrex serial cable. These are a pretty obscure connection, and getting harder to find, but you can probably get one for £10 or so. Note there are some cables which combine these - ie an Etrex connector at one end, and a built in serial/USB adapter.

The old Etrex can be pretty slow to get a position. But should be fairly accurate if you switch it on and leave it for 5 minutes. Then you can mark a waypoint, or record a track etc. Depends on how frequently you want to record different locations, may be easier to leave it on all day.
Depends on just what version of the Etrex it is, should have space for several hundred waypoints.

[Kim]

What sort of GPS precision do you need?  Presumably to within a few metres is fine, otherwise you'd be looking at survey kit.

I'd crack this egg with a modern eTrex and spare AAs as paranoia dictates.  The eTrex 30 supports GLONASS and can average readings over a long period to improve the precision of a waypoint, which works well for consumer kit.

In the absence of that, a modern phone beats an old eTrex in terms of GPS performance (though it may be slower to get a fix if it can't access a network), and an old eTrex wins in terms of ruggedness.  I'd take both, because two is one and one is none.

USB serial dongles work fine with eTrexen, or you can note down the coordinates manually.

[Kim]

** Battery save is in the setup 'system' menu somewhere - what it does is the GPS polls the sats less frequently - typically once per 5 secs instead of once per second - wouldn't be an issue for your purposes in fact I never found it a problem even when following a route on a bike.  Battery runtime is extended by about 40%.  Shame the newer Garmins have dropped this feature.

I expect it was dropped because it no longer saves a meaningful amount of power.

[Biggsy]

It'll be a doddle on the phone, and battery consumption will be hardly anything if you just turn it on for a minute at at each location. 

A suitable app I like is called GPS Test Plus (or GPS Test* for the free version with adverts), from Chartcross Limited.  It can display lat and long in three different formats, and various grid references, as well as confirming whether you have decent GPS reception or not.  It works well on my Sony Xperia Z3 Compact.

* Not to be confused for other apps of the same name from other developers.

[fuaran]

On the phone, I suggest Maps.Me or OsmAnd. Both use OpenStreetMap, and let you download maps for the whole country to use offline. And both will let you mark waypoints, even if the map is blank where you are.

Yes, worth using some sort of GPS status app as well, to check the accuracy and what satellites it can see etc.

[frankly frankie]

I expect it was dropped because it no longer saves a meaningful amount of power.

Even if it only extended the runtime by 15%, I'd imagine a few Edge 800 owners who ride full-value 200s might welcome that ... 

I'd guess that on a modern touchscreen GPS the actual GPS functionality probably accounts for about 30% of the total power consumption (excluding any backlight).  I've seen measurements of older models where it was about 50%.  Actually polling the sats appears to be most of that.  So reducing it to once in 5 seconds really does work.  I used to have it enabled on my little Gecko, which used 2xAAAs and awkward to change, runtime 8-9 hours.  Using battery save pushed it out to comfortably over 12h.  On the road I really couldn't tell the difference, unless I really looked hard for it, eg riding round a roundabout.

[Kim]

I expect it was dropped because it no longer saves a meaningful amount of power.

Even if it only extended the runtime by 15%, I'd imagine a few Edge 800 owners who ride full-value 200s might welcome that ... 

I'd guess that on a modern touchscreen GPS the actual GPS functionality probably accounts for about 30% of the total power consumption (excluding any backlight).  I've seen measurements of older models where it was about 50%.  Actually polling the sats appears to be most of that.  So reducing it to once in 5 seconds really does work.  I used to have it enabled on my little Gecko, which used 2xAAAs and awkward to change, runtime 8-9 hours.  Using battery save pushed it out to comfortably over 12h.  On the road I really couldn't tell the difference, unless I really looked hard for it, eg riding round a roundabout.

New chipsets aren't the same as old chipsets, though.  They likely use much less power, and might lack such power-saving modes because they'd be pointless.  Similarly, the power used to process and write data to memory will have reduced.

Actually... why sit here pontificating, let's measure it...  *heads to the bench*


Okay, here's some actual data:

(click to show/hide)

The plot shows voltage across (yellow) and current drawn^[1] (white) by my eTrex 30, powered via its USB port for convenience (no battery in the unit).  X axis is one second per division.

Spot the point where I turned the GPS reception off entirely ("demo mode" on the eTrex)...

(click to show/hide)

It's the slight peak just before the midpoint of the graticule.  The slight increase in current around that point will be due to rendering the menu.

For comparison, here's the backlight going from 0% to 100%:




And here's the map being zoomed from 200m out to 8km:




Next, I tried to measure the effect of the rate of track logging. I set it to log a point per 99 hours, with the GPS on, and left it on the menu screen to avoid map rendering effects.  Once it had settled to a steady state it looked like this:



I then changed it to record 1 point per second, and again let it settle.  No observable difference.


In summary then, the backlight uses a substantial amount of power (dwarfing all other effects), as does processor-intensive map and GUI rendering.  The GPS receiver itself doesn't seem to affect current draw, either because it's never actually switched off, or because its draw is negligible.

So in terms of battery life, it's mostly about the backlight, and map rendering.


_{[1] Don't read too much into the exact current measurements, I don't have a precision shunt resistor to hand.  They're somewhat high.}

[Wombat]

Thats some seriously detailed responses there, many thanks!

I do already have the Etrex serial cable (just no PC with a serial port), and I do already have OSMand, with Sierra Leone mapping, on my phone.  there will always bwe two of us (its not clever to be alone out in the wilds of Sierra Leone, and my Krio language skills are close to zero, and then maybe any locals may speak yet another language, such as Mende) so hopefully there will be another GPS enabled smartphone available. 

I've never used OSMand for serious navigation, more a case of seeing where on the map I am, and I didn't realise it marked waypoints.  Evening study required!

What of course I failed to mention, and it was pretty stupid not to think of it, is my camera.  The Sony A77v has a GPS, which has helped me locate photos accurately on many occasions in the past    Assuming that is working OK, obviously we will arrive at each location, ensure the camera is powered up long enough to get a fix, and take the photos of the object (could be things like station signs, water towers, odd bits of wall, or even a remote rural station).  Obviously I'd prefer to get two opinions as to location, so it may be camera and phone.  Depending on luggage weight, I may squeeze the tough old Etrex in there too, but now having to take some building survey equipment, that is more under threat than before.  I will not be "processing" any data whilst out there, so its all going to be stored about my person until I get home, then weeks of sitting in front of a PC will ensue.  I will need to be strict on my SD card policy, never go into a potentially dangerous area with an SD card in the camera which carries critical photos, always have an empty one in it.  I expect I won't actually encounter any nastiness, just crushing poverty and deprivation in Freetown.  I'm more likely to be using the (GPS-less) RX100 in an urban situation as its much more discreet, but a GPS location is less relevant there, I'll have a streetmap!

At least its not bloody raining there!  I think a major struggle will be the colours of the country.  The soil is bright red, the trees (where there are any) are dark green, but there seem to be no other colours!  The place names are hilarious, for obvious colonial reasons - I hope to visit Waterloo, Bradford, Wellington, Hastings, and also Mabang, Kissy, and others.

[frankly frankie]

I've never used OSMand for serious navigation, more a case of seeing where on the map I am, and I didn't realise it marked waypoints.  Evening study required!

It doesn't help that 'they' keep improving changing the user interface.  I must admit it leaves me completely clueless.

[Biggsy]

I thought of just taking a photo, but you should check you've got a accurate GPX fix beforehand, with an app like the one I suggested.  You'll see the lat and long at the same time.

I'm a fan of OSMAnd+ for mapping, but it's not the most convenient app for all its functions.

[Phil W]

You can get a serial to USB convertor so your Garmin can be plugged into the USB ports on your puter. I used one before I replaced my old 2002 era Garmin.

[Canardly]

OS locate is a simple location app, a where am I type of thing.

[Somnolent]

I will need to be strict on my SD card policy, never go into a potentially dangerous area with an SD card in the camera which carries critical photos, always have an empty one in it.

Not much changed from the days of FILUM really.
I recall pulling an unexposed roll of 35mm out of the camera, and then the film out of the canister, to keep a gun-toting official from confiscating the camera.
What he didn't realise was that my innocently taken snaps of his country's** sekrit military installations were on the roll in my shirt pocket.

** not SL - but not far away.