Datum's and position formats

payney

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I was hoping some knowledgeable folk would help me improve my understanding of datum's and position formats.

My intention was to go bush (local to me, not very big and I know it quite well) with a 1:25,000 topo map that I purchased from the DPI (VICMAP series) then walk off track until I was away from roads or tracks, Then use my position noted from my GPS and plot my position on the map follow this up with orienting the map and taking a bearing to a feature and navigating to it. I believe this will help me get started on improving my rather poor navigation skills.

My questions arise when I started setting my GPS to the map datum the Australian Geodatic Datum 1966 seen as this datum is in my GPS I thought I would use it I know there are ways of doing this task using the newer (GDA 94) but thought I'd begin my self training this way and learn other methods once I have my head around it a bit more.

So here are the questions,

Is this a good method to begin learning and what position format setting on my GPS will suit the map I am using?
 

biggles1024

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My questions arise when I started setting my GPS to the map datum the Australian Geodatic Datum 1966 seen as this datum is in my GPS I thought I would use it I know there are ways of doing this task using the newer (GDA 94) but thought I'd begin my self training this way and learn other methods once I have my head around it a bit more.

So here are the questions,

Is this a good method to begin learning and what position format setting on my GPS will suit the map I am using?
Hi Payney,

Sounds like a plan. You might like to try performing a resection first and then confirming or correcting your calculated position obtained using the resection with your GPS.
As for the datum, my Garmin GPSMAP 62s, hereinafter called the "62s" has datum named "Aus Geod '66". That would appear to be the one to use.

Cheers,

b.
 

payney

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Thanks biggles I'm not sure I'll be able to try a resection in the bush I will be in as it is quite thick scrub with few notable points and I have a 62s as well it is set for Aus Geod '66 and I'm wondering what to set the position format on to suit the map being used.
 

biggles1024

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Thanks biggles I'm not sure I'll be able to try a resection in the bush I will be in as it is quite thick scrub with few notable points and I have a 62s as well it is set for Aus Geod '66 and I'm wondering what to set the position format on to suit the map being used.
To have your GPS output a grid reference rather than lat and long, you would normally set the position format to UTM UPS, however I'm not sure that would be correct for a map that is set to such an old datum. Is UTM mentioned anywhere in the margin information?
 

payney

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This is the option screen for position formats there are no references to UTM on the map.

10410411_570466616395011_1978442972671772956_n.jpg
 

biggles1024

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UTM on your GPS is third item from the bottom of the list. Use the "Out" button, located at the top right of the buttons to scroll down, one screen at a time. All the formats are listed alphabetically. :) Without UTM on the map, I'd need to see the margin information to have any chance of working out what you'd need to set your GPS to.
 

payney

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Finding UTM on the GPS is easy I wasn't sure if I needed it would a photo of the top left corner of the map give you the info you need?
 

biggles1024

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I can't answer that. I have no way of knowing where the information might be. I would have thought along the bottom edge would be the most likely though. How old is this map? The date the data was correct for that is, not the date you bought it. :_risata:
 

payney

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Haha no need for the cheek it was last revised in 81 there is nothing in the info (the area with magnetic declination and scale and so forth) about its grid only it's datum.

Here is a picture of the top left hope it provides the info you need.

10492556_570475413060798_2883979647565436828_n.jpg
 

biggles1024

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No, that's just the grid. I can only suggest that when you get into the field. Position yourself on a feature such as a road or track intersection or the summit of a hill or a trig point, basically anywhere where you can be 100% certain of you position and locate that position on your map to obtain the Grid Reference. Then read your GPS trying UTM as the position format. It's the only one that I can see that might be applicable and given that your map was produced in 1981, I suspect that it will be correct. Failing that I can only think that you would have to stick with lat and long in whatever format your choose and use the method that was described here in a thread earlier this year (I think but don't quote me on that) that gives you a practical method of converting lat and long into a grid reference. I can't remember the thread title, but a search ought to turn it up. Good luck.
 

payney

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I believe this map uses a UTM grid and I understand how to plot a position now. I have plotted a couple of known points on the map and will navigate to them tomorrow (weather permitting) and then compare the numbers I have written down to my position read from the GPS if they are the same or near the same then I did it right. I will then orient my map and select a feature on the map take a compass bearing and navigate to the feature using my compass rather than the GPS.

I hope I'm on the right track as planning hunting trips and being confident in my navigation abilities in the bush is very important for my future endeavors.
 

biggles1024

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All the best with that and please post back to this thread as I'm interested in how you fare.
 

payney

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Well it went fairly well I did write down the correct coordinates (or near enough) to what the GPS read in that location. However after forgetting to print and take the Geoscience Australia map reading guide I could not remember how to correctly orient the map and allow for magnetic declination so my compass bearing was out by around 70 meters I did still find the feature though and safely return to the cruiser it was an enjoyable little trip wandering off track very exciting came across a few things on the way to.

This is the type of country I was walking you can see the light rain in the photo.

10404229_570982019676804_6940366517189324304_n.jpg

This sword grass would have done a decent job of natural cordage if the need had of arose.

10341487_570982049676801_5157702643852834798_n.jpg

This is a ... I know nothing about these things but thought I'd snap a pic feel free to fill me in if you know anything on the subject.

10477048_570982206343452_2917662743334183644_n.jpg
 

Bloffy13

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Payney,
Good stuff.
There are other formats available such as MGRS but UTM is the most common.
I don't know about your GPS but mine, an earlier Etrex, will work out the mag variation and shows it.
Being WA it's only about 30mils anyway so usually I ignore it unless the legs are over a longer distance or absolute accuracy is essential.
I tend to use the aim off method anyway. That is, if I am aiming for eg a track junction, I aim just off the point (say to the right) to hit the track. That way when I hit the track, I know I need to turn left and move to the junction. It's a lot easier then to locate a set point. If you aim directly at it and then the point is not obvious when you arrive, do you turn left or right?
Try out some geocaching. It's a good way to get to know, and trust, your GPS. If you can then do it map to ground, even better.
Cheers
Bloffy
 

payney

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Thanks bloffy I have done a bit of geochaching and it helps I have a fair amount of confidence with my GPS but I want to learn more on the map side of things it's a slow process as I don't have a lot of spare time available but I will get there I printed the geoscience Australia map reading guide and I will read through it a few times try and make it sink in.
 
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