Differences Between WGS & NAD Map Datum~How They Effect Maps

[maxpowers]

If you are using GPS: Be careful. GPS is based on the map datum: WGS84. Which means "World Geodetic System ~ 1984". What this means to your GPS is the co-ordinates of the land are based on the year 1984. So if you programme in a direct route from point A to point B it may pick a road that was deactivated in 1985. That road could be very dangerous. Remember, Common Sense.

Don't go with falco you will be lost in no time!

wgs84 has nothing to do with when roads were built used and deactivated.

it is the elliptical shape chosen to be the most accurate representation of the earths raw surface elevation.
the earth is not a perfect sphere so this is the egg model they use.

cheers

[FalcoColumbarius]

WGS began replaceing NAD27 (in N.A.) as a map basis because the terrain & technology has changed so much since 1927 (for instance: man made lakes and the use of satellites), as well as creating a world wide datum based on satellite data rather than the old terrestrial grid system that changed from continent to continent due to national interests . There have been a number of WGS incarnations (60, 66, 72) The most recent has been 1984. There was an NAD83 datum which uses a different measuring system than NAD27 and many maps are using the NAD datum but Global Positioning System is better suited to the WGS system due to the satellite technology, as I understand it. WGS84 was last revised in 2004 and due to be revised this year (2010) ~ but they don't necessarily get everything. Also, just because a road is deactivated doesn't mean that it is no longer there.

The people at Garmin first informed me about this (as that is the system I use).

Maxpowers has made a point: You don't really know who I am ~ and I can guarantee you that I am not infallible. If you wish to investigate further (which I recommend you do) then Wikipedia is a good place to start.

Falco.

[Green1]

Falco: just a quick note on map datums, WGS84, NAD27, etc... they only account for your position on this round ball we call earth, they have nothing to do with features such as lakes, mountains, roads, etc. None of those are included in the datum itself, only in the maps themselves.
You can find perfectly accurate, or completely inaccurate maps in any datum, the only really important part about datums is making sure that the map you are using matches the datum you are using (eg, if your paper map is WGS84 and your GPS is set to NAD27, you could easily find yourself plotting your position half a kilometre out!)
This becomes even more important if you are relaying coordinates to someone else, if you don't know what datum their map uses and you don't specify, it is easy to end up a long way out from where you should be. (people commonly give lat/long coordinate read off their GPS that would indicate a position within a few cm, and yet don't include a datum making the position useless for accuracy less than several hundred metres)

Just because you use a "newer" datum will make absolutely zero difference on how a map plots the course of a river, a lake, or a road. that is a separate issue entirely.

[FalcoColumbarius]

Falco: just a quick note on map datums, WGS84, NAD27, etc... they only account for your position on this round ball we call earth, they have nothing to do with features such as lakes, mountains, roads, etc. None of those are included in the datum itself, only in the maps themselves.
You can find perfectly accurate, or completely inaccurate maps in any datum, the only really important part about datums is making sure that the map you are using matches the datum you are using (eg, if your paper map is WGS84 and your GPS is set to NAD27, you could easily find yourself plotting your position half a kilometre out!)
This becomes even more important if you are relaying coordinates to someone else, if you don't know what datum their map uses and you don't specify, it is easy to end up a long way out from where you should be. (people commonly give lat/long coordinate read off their GPS that would indicate a position within a few cm, and yet don't include a datum making the position useless for accuracy less than several hundred metres)

Just because you use a "newer" datum will make absolutely zero difference on how a map plots the course of a river, a lake, or a road. that is a separate issue entirely.

Yes, thank you. I caught that the first time when Maxpowers brought it up, which is why I elaborated on my earlier comments. In fact you could plot yourself out by a degree ~ westward in my experience of comparing co-ordinates with a different map datum. Still, from what I understand the World Geodetic System is based on satellite technology (ball/global) rather than the terrestrial grid/plate model of NAD. Maxpowers mentions:"... it is the elliptical shape chosen to be the most accurate representation of the earths raw surface elevation."... According to Wikipedia: "The coordinate origin of WGS 84 is meant to be located at the Earth's center of mass; the error is believed to be less than 2 cm."

My original point is not about the fact that it is a new way of measuring, but that the new system was originally put into place in 1984, consequently the info that they put into that system was originally based on 1984 data* (not to be confused with Map Datum). WGS84 has been revised in '96 & '04. Map features are revised periodically. A deactivated road can change dramatically between seasons, let alone decades.

An example of this would be when I split up from the rest of the Fry Lake crew and went exploring on my own ( http://www.delica.ca/forum/another-story-3857.html ). I had entered the "Most Direct Route (including dirt roads)" from where I was to Coombs. My GPS found a road called the Gilson Road. According to my GPS it was a dirt road. According to my observations it resembled where there used to be a road. It was so dilapidated that I would often get out of the van to survey or make modifications to the road so I could get past that point.

So yes, I am aware that a map datum has little to do with how the cartographer wrote the map in respect to terrestrial features, but the features may not have all been revised in the last twenty-six years since the advent of WGS84.

Falco.

[maxpowers]

Gps uses the wgs84 as the round elevational model to plot nad83 utm coordinates on. the Wgs84 elliptiod shape model was needed because Gps works in 3d, and all previous maps are only 2d. it needs the average surface elevation or it freaks out.

the wgs84 model, is a completely blank roundish globe, thats it, there is nothing on it.

you can add whatever you want. utm coordinates and then information, roads, forestcover, mineral deposits, population. cities. all these are added and changed whenever by whomever for what ever.
all these things are layered on to the base model. but the base model which is wgs84 with what ever utm. will never affect what roads appear active and such. that is just map software needing to be modified.

hope this helps.
cheers Jeff.

[Green1]

My original point is not about the fact that it is a new way of measuring, but that the new system was originally put into place in 1984, consequently the info that they put into that system was originally based on 1984 data* (not to be confused with Map Datum). WGS84 has been revised in '96 & '04. Map features are revised periodically. A deactivated road can change dramatically between seasons, let alone decades

but the 2 are completely unrelated, there is NO data about land features at all "in" either one, depending on your particular cartographer you can have a BRAND NEW map made on NAD27 with perfectly accurate information, or a 26 year old map on WGS84 with everything out to lunch. It doesn't matter what the datum is, it only matters that your consistent across all your uses.

Sure WGS84 theoretically allows a cartographer to make a better map, but it doesn't matter unless they actually DID.

In actual fact, it doesn't matter one bit which datum you are on, it only matters that your GPS and your map match. I have frequently been involved in situations where 2 different organizations are using 2 equally new and up to date maps from different sources using different datums. You just need to make sure you know which you are on so that when you communicate it you can account for any differences

[FalcoColumbarius]

Green, did you actually read the paragraph that you quoted out of my post or did you just grab something that vaguely looked like it was at odds with your point of view?

....consequently the info that they put into that system was originally based on 1984 data* (not to be confused with Map Datum)....

Information on a map, like mountains, roads, fish farms, &c., is also known as data (information; input; the measurements; the picture; the statistics; the details; the bottom line.....). My reference to WGS84 was not about the system itself ~ but when they introduced the system they made new maps (GPS & otherwise) and not everything in those maps is completely up to date. I agree, WGS is a measuring system based on a three dimensional model of a sphere, this would explain why I get elevation with my map placement. But I also understand that not everything on my map is completely up to date, as I have discovered in theory and in practice, both on my system and also finding young families in their Honda Accord lost on a deactivated road because their talking GPS unit told them to go there.

Same with the Mussio Ventures Back Roads Map Books ~ there are thick black lines (FSRs) in those books that in reality look like dried out creek beds.

We have gone way off topic. This thread is about Ursula asking for advice regarding going off road. If you want to start a new topic on what a map datum is ~ then have at 'er.

By the way, Ursula ~ I also keep an old transit compass with me, too.

Falco.

[Green1]

And you did not read mine.
I can show you a NAD27 map more up do date then certain WGS84 maps. The system is IRRELEVANT.

As for "they put more data in to it"... no, they didn't, they didn't put ANY data in to it AT ALL.

I can have a NAD27 map based on 2010 data just as easily as a WGS84 map based on 2010 data.
Which is more up to date? a map based on information before 1984, but updated regularly to current day, or a map based on post 1984 data that hasn't been updated in 10 years?

Just because something is WGS84 doesn't make it any more accurate, or any more recent information than one based on NAD27.

The sooner you understand the difference between datum and data, and how there is no correlation or connection, whatsoever, in any way, shape, or form, between the 2, or even an implication, inference, or likelihood, of one affecting the other. The sooner you will understand that the datum only matters if you are comparing 2 different sets of coordinate systems. As long as you are consistent in your usage there is no advantage to you whatsoever to use WGS84 over NAD27 or vice versa. both will be perfectly accurate as long as the map itself is accurate, and the map will be accurate if it is recent and well researched, regardless of which datum it is using.

[FalcoColumbarius]

The system is IRRELEVANT.

NAD27 is a two dimensional/plate model. WGS84 is a three dimensional/spherical model. I would say the system is very relevant.

In 1983 there was an incident over restricted Soviet airspace. A Korean air liner got shot down because the auto-pilot kept drifting in and out of restricted Soviet airspace. 269 People died. After this incident the President of the US decreed that GPS would become available to the general public. In 1984 the system was modified. GPS was becoming a public domain. They made new maps.

Falco.

[Green1]

The system is IRRELEVANT.

NAD27 is a two dimensional/plate model. WGS84 is a three dimensional/spherical model. I would say the system is very relevant.

Do you go offroading with a globe? or a local map?
over the distances on a local map, it's irrelevant.

In 1983 there was an incident over restricted Soviet airspace. A Korean air liner got shot down because the auto-pilot kept drifting in and out of restricted Soviet airspace. 269 People died. After this incident the President of the US decreed that GPS would become available to the general public. In 1984 the system was modified. GPS was becoming a public domain. They made new maps.

When and why GPS was made is irrelevant to the discussion. GPS again has NO information about land masses, rivers, roads, or any other feature, only your location on this planet. as for "they made new maps"... who is "they"? every map is made by a different company, and most use data much newer than 1984... and surprisingly (for you) many still put that NEW data on a NAD27 datum... because the DATUM is UNRELATED to the DATA. Each cartographer does things differently, the DATA is not "public domain" and there is no magic 1984 view of the world with all the features on it that is being used here.
Just the SHAPE of the globe.

you MUST use the same datum that your map uses, no matter what. And disregarding a perfect map because you don't like the datum would be a very silly move indeed!

Many maps are still published in NAD27, I'm not going to avoid them to get a WGS84 map that is WORSE!

The DATA is serperate from the DATUM, you just don't seem to understand that the 2 are completely, 100% unrelated.

Think of the datum as a piece of paper, if I use a 1927 piece of paper, or a 1984 piece of paper has no bearing whatsoever on the map I draw on it with my pen. the paper is blank until the cratographer draws on it.

[jwfchase]

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[Green1]

If you look up at the top of the page you may notice that we are already IN the "new" thread...

[jwfchase]

Oops... sorry!

[marsgal42]

Maybe I'm just being old fashioned, but there is something to be said for seeing where you're going...

Straight in approach to CYNJ runway 19.

CYNJ approach.jpg (65.39 KiB) Viewed 4751 times

...laura

[psilosin]

Laura...your Delica is lifted REALLY high to have a view out the windshiled like that!!!