Garmin Foretrex 401 : ITM (Israeli Transverse Mercator)

[Locus]

EDIT : Solved. Skip to Post #10 in this thread, for ITM Settings (2014)

ITM / Israeli Transverse Mercator (More Info) is the Grid on 1:50,000 Topo maps I will be using for some hikes in Israel.

Getting around will be in the usual manner (Map & Compass).

It would be fun to bring along a recently purchased Garmin though, for marking interest points, water sources, etc.

An example location on one of the maps would be - 251128  / 791784  (E/N)

ITM doesn't show up in the list on my ForeTrex 401 unit, so I'm entering it manually in the hope this will work.

Instructions I've found online for setting up a GPS unit to ITM, are below in Black & Green :

Long. of Origin: 35° 12.724 E
False Easting: 220251m
Scale: 1.0000067
False Northing: -2885259m
dx: -235m
dy: -85m
dz: 264m
da: -164m
df: -0.547

Keep in mind that I'm a complete newbie in this area of my Garmin.

Below is how things now look on screen after my data entry into Setup -> Units -> Position Format -> User Grid

Longitude Origin : E 035°12.724'
Scale : 1.0000067
False E  (MT) :  22025100.0
False N (MT) : -2885259.0

Followed by how it looks on screen after my data entry into  Setup -> Units ->Map Datum -> User

(Btw, WGS 84 appears here by default until I choose 'User'. If this is relevant or not, I don't know)

dx (MT) :  -235
dv  (MT):   -85
dz (MT) :   264
da (MT) :   -164
df :    -0.0000547

I began to doubt if I entered numbers in the right places, or perhaps too many zeros after/ before.

For example, the df field and the False E / False N fields.

If it will help to determine that, below is how each field looks when I'm entering data into each field.

Longitude Origin : E 035°12.724'
Scale : 01.0000067
False E (MT) :  22025100.0
False N (MT) :  -2885259.0
-----------------
dx (MT) : -0235
dv  (MT) : -0085
dz (MT) : 00264
da (MT) : -0164
df : -0.0000547

Once I create this custom setting while in Britain where it would be SH 71293 12380 for example, it looks like.....

E-------- (8 Blank Dashes)
N-------- (8 Blank Dashes)

My 'assumption'  is that once in Israel the unit will show E251128 / N791784 or similar?

Assumption is a gamble that could leaded to wasted time once there if I'm wrong, so I'm seeking second / third opinions.

Many thanks 

[captain paranoia]

I'm not sure how Garmin's user-defined grid entry works, and there's no explanation in their wonderful manuals...  I think all the d values (deltas) are to cope with the different datums (WGS84 for GPS, GRS80 for ITM).

But it does look like the false E and df values are incorrectly entered.

I imagine the reason you see E-------- N-------- is because the position in the UK using this grid system is beyond the numeric range of the display (hmmm... 8 digits, assuming they're metres, would give 99999.999km; that's more than the circumference of the Earth, so we cannot be that far from Israel; we're probably negative E, though).

Where did you find the instructions for setting up the Israeli grid?  The numbers you've given don't seem to agree with the Wiki entry on ITM.

I confess that I'm struggling to understand how you can specify a local grid system without needing to specify both lat and long of the grid origin (from where you centre the Transverse Mercator projection, and apply a false origin offset).

If you had an Android smartphone, and used OruxMaps, this thread might be useful:

http://www.oruxmaps.com/foro/viewtopic.php?f=7&t=1618

[Locus]

Hi captain paranoia,

Much of it has been experimental with the 401 really, after getting information on the Grid.

I agree that the False E & False N look suspect, after me entering the data.

I'll fiddle around with those a bit and see how they appear.

The numbers you've given don't seem to agree with the Wiki entry on ITM.

Very different, yes. Only the Grid Scale Factor matches.

I imagine the reason you see E-------- N-------- is because the position in the UK using this grid system is beyond the numeric range of the display (hmmm... 8 digits, assuming they're metres, would give 99999.999km; that's more than the circumference of the Earth, so we cannot be that far from Israel; we're probably negative E, though).

My guess is similar, and my gamble has been that once in situ it will display (if' my User Grid information is correct, that is).

My source for the version of ITM given here is from a (website) dedicated to one of the smaller trails in Israel called the Jesus Trail, and the page has information for setting up a GPS to both the ICS (Older System) and the ITM (New System), as the topo maps tend to show both. One other link shows similar data for ITM, the only difference being that the Scale is 1.0 (which is the data for ICS, the old Grid) rather than 1.0000067. The information from the Jerusalem Trail comes across as confident and I've been trusting that for now as it is an operating website, but it is of course possible they just pasted data from an older source, the web being what it is. I've read some sites which suggest you can create a waypoint once in a country and then work around things manually. All of this is a learning curve

[Lyle Brotherton]

I have been pondering your dilemma.

There are two ways you can remotely, as in not physically being there, test your false origin settings:

1.   Using a local ITM map, as these have latitude and longitude marked on them, however it is important to remember that when you draw these onto the map itself they will not run parallel to the ITM grid lines and you will need to align them with the Lat/long graticules marked on the map.

i.     Select a small feature on this map
ii.    Measure both the ITM grid reference and take the Lat/long coordinates of this feature
iii.   Fire up your Fortrex
iv.   Set the units to Lat/long
v.    Enter the Lat/long coordinates of the feature as a waypoint
vi.   Set your Fortrex units to the ITM Grid with your false origin settings
vii.  Open your new waypoint and compare the ITM grid reference to that of the datum

If the two are the same your Grid settings are correct

2.   Cross referencing datum.

MAPI (Mapping Center of Israel) publish reference coordinates along the ITM central meridian, which crosses thru Jerusalem, using ITM Eastings and Northings plus the Lat/long equivalent.

i.    Fire up your Fortrex
ii.   Set the units to Lat/long
iii.  Enter the Lat/long coordinates of the published datum as a waypoint
iv.  Set your units ITM using your Manual Grid with your false origin settings
v.   Open your new waypoint and compare the ITM grid reference to that of the datum

Similar to the previous technique, if your displayed reading matches the quoted reference, then your Grid settings are correct.

[captain paranoia]

I had a look around Garmin's website, and a number of their user manuals.

And I could find no useful description of user datums, or user grids.  The manual for the Foretrex 301/401 is spectacularly devoid of specifics of how the various Settings work...  The manual for the old 12XL is somewhat more informative.

Why have a feature on your receiver and then not explain anywhere how it should be used?

I also find it noticeable that Garmin don't seem to provide datum/grids for either of the Israeli systems.  Which, given the close relationship between the US and Israel, seems a little odd...

Lyle, do you have any contacts at Garmin who might be able to explain how this feature should be used?

[Locus]

Many thanks for everyone's input. Quick update to let you know that your posts were read earlier and there has been some good developments with a Northing on my unit after using a variation of Lyle's suggestion. Easting is roughly 200m out though which is puzzling, so there is still some  tweaking to be done to find out why.

I'll explain further on another post later today.

[Lyle Brotherton]

Looking forward to hearing more about your quest Locus.

I have a suspicion that your Easting discrepancy may be to do with the different geodetic datum you are using, as asserted by CP,  because although the GRS80 (Geodetic Reference System 1980) was originally used by the WGS84 (World Geodetic System 1984), the reference ellipsoid of WGS84 now differs slightly due to its later refinements.

[captain paranoia]

Taking Lyle's excellent waypoint suggestion; most maps with grids also identify the lat/long of the corner(s) of the map.  These would make good example waypoints, since they would need no interpolation of grid or angular axes.  I'd advise testing a number of points, to make sure your user grid isn't distorted compared to ITM.

Then we're into the problem of how precisely the lat/long is expressed on the map; an arcsecond subtends about 30m (for Northing, varying with latitude for the Easting).  So, to get down to metre accuracy, you'd need to have lat/long specified to a precision of 0.03 arcseconds...  DD:MM:SS.ss

Oh, and I forgot to say: the df entry does look like it's intended to include a scale factor of 10,000, but the value displayed includes that scaling.  So you enter -0.547, but this is accepted and reported as -0.0000547.  Perhaps...

[Lyle Brotherton]

How have you got on Locus?

[Locus]

Morning Lyle.

For now it could be said to be around 60% sorted. Later on, 'another' set of values to input into a Garmin for use with ITM was found online.  It was noted down for future reference but as value for the false northing was very different from the other, my guess at the time was that it was an older setting. Eventually, those more recently found settings were, quite to my suprise, confirmed by someone in Israel as being the ITM settings which he has been using with Garmin and the Maps for several years now, and has found to be spot on. From that point on, my attempts at testing this remotely in a 'manual' way, well that has been amateur and fumbling to put it mildly, leading to mixed results that may be explained by my current minimal understanding of Lat / Long. It is something to be worked on.

In the meantime, my 'cheat' attempts have been using online mapping to make a waypoint and then converting that to ITM settings entered into my unit. Currently there is an error of about 150m on the Northing, and a Prefix of 3 appears on the northing of my unit, which is a mystery. In regard to an earlier mention of it being unusual that ITM is not included on Garmin with others, one of my early guesses was also that it could be due to security reasons but that later seemed unlikely given that the entire mapping with grids is available online, paper topo maps are easily purchased in Israel or by airmail from overseas and there are Grid systems for other far less sensitive countries in the world which are also not included on Garmin either. All I have heard about Israel in regard to GPS and security is that sections of the Southern Negev can be subject to jamming devices. Garmin pre-installed Datums may just be popularity based.

[Locus]

Updating this and also for anyone who may browsing the web for ITM info :

I was only able to get these settings confirmed by one person over in Israel, but.....

Inputing User Grid and Map Datum for ITM (for a Garmin Foretrex 401, at least) in 2014, for use with SPNI 1:50,000 Maps :

(-) Setup
(-) Units

(-) Position Format
(-) User Grid

Below is how each field should look while entering the data into the unit.

-------------------------------------------
Longitude Origin : E 035° 12.725
Scale : 01.0000000
False E (MT) : 00220251.0
False N (MT) : 00114750.0
-------------------------------------------

(-) Map Datum
(-) User

------------------------
dx (MT) : -0235
dy (MT) : -0085
dz (MT) : 00264
da (MT) : -0163
df : -0.5439085
------------------------

Creating a UTM waypoint on my Garmin with WGS84, and then converting it to the above User ITM Grid and Datum:

36 S 0736715
UTM 3664112

E00250447
N03777233

Firstly, the prefix of 3 before the Northing remains a mystery but the person over in Israel told me he also gets that 3 Prefix on ITM Northings with a Garmin unit ((Etrex Vista) but no mention of problems. We don't why it shows that way as the ITM Northings shown on the maps or online, would be 777 or similar, not 3777. If it proves to be problematic once on the ground in Israel, this post will be updated. For now it seems to be a harmless oddity of Garmin with User ITM Grid / Map Datum. 

Image #1  : Building chosen (marked in red) about 10m across.



Image #2 : On 1:50,000 SPNI Map #1



Image #3 : Using the Map Romer Scale Grid Reference Tool for 250447 / 777233



For my purposes, that will probably do fine. However..........

If my ' eyes'  are correct, does that final easting number of 7 land a bit too far to the east of the chosen building?
I assume (incorrectly?) that once splitting and counting the square east again after the second 4, count resumes from 0?

[boogyman]

Hi Locus,

Using a ruler (on image #3, measuring directly on the PC screen) I would say that the 447 looks good. As a matter of fact, if I would not have known the coords upfront, I would have "estimated" the center of the red building as 450 (again, just by measuring on the PC screen).

Best regards,
Chris.

[Lyle Brotherton]

Locus, one of the more esoteric posts on this forum, and probably not for everyone, but I have most certainly learned a lot from it - your description of how to set up the false origin is really straight forward and easy to follow.

I made the same on-screen measurements as Boogyman, was he, are we, we correct?

[Locus]

Good Evening Chris and Lyle. 

Yes, it seems that my earlier eye placement of that last 7 arose from a misunderstanding of how to read things once it gets that fine, but the penny has dropped now after both your posts. Cheers for that. It has been interesting for me too, and sparked a side interest about getting to know UTM more, and Lat/Long (eventually). Many thanks for everyone's assistance and input on this one.