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ITM Grid and Map Datum for GPS

 
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Hummus



Joined: 07 Mar 2014
Posts: 24


Location: Britain

PostPosted: Sun Mar 23, 2014 12:56 pm    Post subject: ITM Grid and Map Datum for GPS  Reply with quote

Has anyone been using the ITM (Reschet Jisrael Hadascha  רשת ישראל חדשה) Grid on the 1:50,000 Maps in conjunction with GPS?

As the INT sections of the maps in the guidebook are excerpts from the SPNI Maps, the margins are missing in 'most' cases but some show it (Map 4, for example), so I managed to manually write the Eastings and Northings of the ITM Grid onto each grid on each map page by following the trail and utilising amudanan so that a GPS can be used directly on the trail for location and waypointing in relation to the maps. For example, one location could be :

MAP No.4

E00251463
N00776118

However, now I'm finding discrepancies about setting up a Garmin GPS to the ITM Grid and Map Datum.

One online source (Jesus Trail Website) says :

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

One other source (Rechasim) says :

Longtitude Origin: 35° 12.725 E
False E: 220251.0
Scale : 1.0000000
False Northing: 114750.0
------------------------------
dx: -235m
dy: -85m
dz:  264m
da: -163m
df: -0.5439085

Various websites and authorities in Israel were emailed, but no response yet.

If anyone in Israel can confirm which (if any) of the above settings are accurate for wiring a GPS to the ITM Grid for use with the maps, that would be great. According to some web browsing, it seems that GPS units sold in Israel may already have the ITM Grid and Map Datum included in the settings? , but in my case this has to be entered manually as : Position Format -> User Grid and Map Datum -> User Grid and this is where I need your help.

Maybe the SPNI offices in Israel or a GPS shops in Israel will have the settings?
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yankale
Site Admin


Joined: 27 Oct 2008
Posts: 397



PostPosted: Sun Mar 23, 2014 3:39 pm    Post subject: Reply with quote

Pls. check here: http://israeltrail.myfastforum.org/about172.html
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Hummus



Joined: 07 Mar 2014
Posts: 24


Location: Britain

PostPosted: Sun Mar 23, 2014 11:37 pm    Post subject: Reply with quote

Hi Yankale. Many thanks but the site only deals with local magnetic variation.

Is it possible that Israelis on the Hebrew Forum would know the settings?
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yagil



Joined: 19 Nov 2013
Posts: 36


Location: Israel

PostPosted: Mon Mar 24, 2014 12:21 am    Post subject: Reply with quote

Hi Hummus,
Personally I've been using the Rechasim parameters for the last several years with old and new Garmins and they seem to be spot-on. I'd give them a go.
Now just be aware that Israeli maps have three sets of numbers: UTM (Blue), old Israel grid (Black) and new Israel Grid (Red). You need the red ones.
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Hummus



Joined: 07 Mar 2014
Posts: 24


Location: Britain

PostPosted: Mon Mar 24, 2014 4:07 am    Post subject: Reply with quote

Many thanks, Yagil. It appears you have used magic

Prior experimentation with the two versions was giving mixed results over recent days.

As I am still in Britain, I was testing them remotely by choosing a recognisable feature on satellite imagery which also appears on the guidebook map excerpts, then creating a waypoint with its Lat & Long on my GPS then coverting this waypoint to the ITM parameters being given. Until now, only the JT parameters were bringing up recognisable Grid numbers but accuracy would fluctuate on different location tests, sometimes up to 40 mtrs off. Rechasim parameters were not giving a recognisable Grid reading, but now do this evening and explains why I think you used some magic Yes

Test with Map #4 :

N 33° 05.378'
E 035° 32.728'


I chose the easterly circular tower that I've marked with a green dot on the images below.

Images :  1 (Sat) - 2 (Map) - 3 (Romer)

My GPS shows ITM Location :

E00251377
N03777244

Much much better. Easting is a bit too far to the East of the tower though.

I get a better Easting result of E00251314 if I change the Longitude Origin to E 035° 12.765'

(but perhaps doing so may cause problems on other locations. I will experiment).

On the Northing, the appearance of a 3 prior to the 777 is a real mystery though

Any ideas on that? Maybe I am entering the parameters incorrectly.

If I may, can I check if you enter parameters the same way as I do, Yagil?

Below is how the screen appears when I'm entering them into each field on my Garmin GPS.

Position Format > User Grid

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

Btw, yes the different grid systems on margins. Because of that, I manually wrote ITM Eastings and Northings next to each grid of each map margin so it would be quick and easy to read once out on the trail. Interested to hear if you know what may be causing that 3 to appear before the Northing  
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yankale
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Joined: 27 Oct 2008
Posts: 397



PostPosted: Mon Mar 24, 2014 10:16 am    Post subject: Reply with quote

This is a nice thread
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Hummus



Joined: 07 Mar 2014
Posts: 24


Location: Britain

PostPosted: Wed Apr 02, 2014 11:48 am    Post subject: Reply with quote

Good Morning Yankale and Yagil,

Updates on this are this are that my several experiments have led to success by doing a remote conversion from UTM location to ITM Grid on the maps. For location accuracy, this now seems perfect for my needs. Now the only think I would like to get clarified before putting this matter to rest is the unusual 3 that appears on the Northings on GPS. Can you confirm if your GPS shows a 3 prior to the Northing on an ITM reference?

If it does, then all is well. If not, then maybe this could cause incorrect waypoint records?

One example location is below, first in UTM then the same in ITM.

36 S 0737578
UTM 3664106

E00251309
N03777209

Cheers  
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yankale
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Joined: 27 Oct 2008
Posts: 397



PostPosted: Wed Apr 02, 2014 12:01 pm    Post subject: Reply with quote

Hummus,

I hope Yagil is able to respond soon. He is on a field trip until late Thursday.

 
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Hummus



Joined: 07 Mar 2014
Posts: 24


Location: Britain

PostPosted: Tue Apr 08, 2014 12:44 pm    Post subject: Reply with quote

Hi Yankale. Unfortunately it seems Yagil may have missed the post.

Do you happen to know of any other sources who may know the answer?
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yankale
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Joined: 27 Oct 2008
Posts: 397



PostPosted: Tue Apr 08, 2014 1:08 pm    Post subject: Reply with quote

No I don't, but I've asked Yagil to comment. He will be back tomorrow night and will answer you then.
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Hummus



Joined: 07 Mar 2014
Posts: 24


Location: Britain

PostPosted: Tue Apr 08, 2014 1:14 pm    Post subject: Reply with quote

Ok, thanks for that Yankale 
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yagil



Joined: 19 Nov 2013
Posts: 36


Location: Israel

PostPosted: Wed Apr 09, 2014 4:49 pm    Post subject: Reply with quote

OK, I think we have a solution here. In the Rechasim Datum, the DF should be -0.54739085.
Try it and tell me if it helped something.

Also, My GPS (an old Etrex Vista) doesn't give me a 0, but it does give me the 3.
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Hummus



Joined: 07 Mar 2014
Posts: 24


Location: Britain

PostPosted: Wed Apr 09, 2014 6:15 pm    Post subject: Reply with quote

Hi Yagil,

Giving that a try, I discovered that on my particular unit the maximum numbers it allows me to enter in the DF datum field after the -0.  is seven numbers so it won't allow me to enter the -0.54739085 (8 numbers after the -0. ) in full. As an experiment I entered 0.5473908 and the result ITM of converting from my UTM waypoint comes up with same ITM location from the earlier DF value of -0.5439085. Both give me the following :

E00251309
N03777209

From that it seems that the extra 5 or even other alterations to the DF must only be tiny and barely perceivable for general navigation. It may be that, as you suggest, some GNSS units will show a 3 prefix on the ITM Northing for some reason and others won't. If your Garmin unit also shows a 3 prefix on the ITM Northing and you have found that this still works fine for waypoints and other GPS use, then this is a good sign I think

I believe that the Position Format and Map Datum entry on Magellan units can sometimes have very long fields of entry for many more numbers compared to Garmin, and this may explain the differences in the appearance of data entry values for ITM found on the web. From this point, this is probably as far as I can go until arriving in Israel and experimenting. Many thanks for the help of both yourself and Yankale  

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