I love accuracy... so why..... ????

[+Desert_Warrior]

I love accuracy. In my former career, it was necessary. Anybody know what "Danger Close" means? Now I shoot high-power rifle and combat pistol as hobbies. I was always content with my collection of military maps, till now.

 

I notice that most GPS units default to WGS-84. I see that most geocaching is done on WGS-84. My mil maps are NAD-27. Most of the USGS topo maps I have picked up are NAD-27 with tick-marks for NAD-83.

 

So.... am I being nit-picky here? I know my military maps are O-L-D !!! But my USGS maps are 1995 or better. How are you handling this, or am I the first one to notice?

 

I eagerly await the combined knowledge of the group.

 

Mike-KD9KC

[+EraSeek]

Yes, most usgs maps are still in NAD27. WGS84 and NAD83 are, for all intents and purposes, the same. You can input coords from your NAD27 map into your GPS by first switching your Map Datum setting in your GPS to NAD27 > then input the NAD27 coords > then switch your setting back to WGS84 if you wish. Or the other way around, take your WGS84 coords in your GPS and just switch the setting to NAD27, and you have the coords that match your map.

 

[jfitzpat]

As mentioned, NAD-83 and WGS-84 are essentially the same.

 

NAD-27 is, thankfully, becoming a thing of the past. If you love accuracy, I would avoid NAD-27 except when your printed map has no other scale.

 

-jjf

[Kerry.]

There's no consistent relationship between NAD27 and the real world, which is why some GPS manufacturers have to implement many "different" NAD27 datums (most Garmins have for instance 11 "different" NAD27 options) depending on the location.

 

There's also no common set of conversion parameters which fit the bill which is why distortion models are used in most conversions these days.

 

Hopefully in due course ALL maps/charts will be in one uniform based datum and they won't be anything as ancient or outdated (or inaccurate) as NAD27.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[Kerry.]

There's no consistent relationship between NAD27 and the real world, which is why some GPS manufacturers have to implement many "different" NAD27 datums (most Garmins have for instance 11 "different" NAD27 options) depending on the location.

 

There's also no common set of conversion parameters which fit the bill which is why distortion models are used in most conversions these days.

 

Hopefully in due course ALL maps/charts will be in one uniform based datum and they won't be anything as ancient or outdated (or inaccurate) as NAD27.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[+Kouros]

How do people deal with OS grid references?

 

OK, so it only really applies in Britain, so I guess I'm in the minority here, but still... anyone else have that problem?

 

------

O God, I could be bounded in a nutshell, and count myself a king of infinite space, were it not that I have bad dreams.

 

Hamlet, II.2 252-253

[+Macro]

For the benefit of us who arent trained as cartographers, what is the actual difference in the different standards? Also, when referring to "datum", exactly what are you all referring to. I would love to understand this a little better and this seems like a good place to break it completely down.

 

Also, still waiting to see what "danger close" means.

[jfitzpat]

quote:

---

Originally posted by macro:

For the benefit of us who arent trained as cartographers, what is the actual difference in the different standards? Also, when referring to "datum", exactly what are you all referring to. I would love to understand this a little better and this seems like a good place to break it completely down.

---

 

Basically, the problem is that the earth is, geometrically, a form of 'spheroid' - a slightly flattened sphere. A spheroid is an 'undevelopable' shape, that is, there is no way to unfold it to lie flat.

 

The implication is that there is no way to simply 'flatten' the earth onto a printed map and maintain accuracy regarding distances, relative bearings, etc.

 

So, cartographers use 'projection systems' to flatten the earth onto maps and try to preserve some semblence of accuracy. There are lots of projection systems, or mapping datum - and they use different mathmatical methods depending upon their use (cylindrical, conic, planar, etc.) - ie, what mapping aspect they are trying to keep the most accurate.

 

The problem with NAD27 is that the model is old and flawed. You basically calculate a result, then apply a century of empiracally gathered offset data to correct for problems with the model.

 

-jjf

[+TheGertridgeExplorers]

Also, don't mix projections and datums together. A datum is a mathematical model of the shape of the earth. NAD27 and NAD83 are datums defined by ellipsoids. Actually NAD27 is Clarke's 1866 ellipsoid tangent to the earth at Meade's Ranch, Kansas and oriented in a specific direction. NAD83 is the GRS80 ellipsoid and is earth-centered.

Projections are the way a point on the earth is transferred to a flat piece of paper. Imagine wrapping a flat piece of paper (map) around a slightly squashed basketball (the datum) in different ways (projection). Cylinderical and Conical are the most common. Then imagine shining a light from the center of the basketball and drawing points on the paper where the points on the basketball are "projected". Now you have a map. So you can have a conical(Lambert) or cylindrical(UTM) projection in NAD27 or NAD83 and the coordinates can be given in feet, metres or lat/long.

Gee, I guess maybe I didn't sleep through all of those boring classes at university! I hope this makes things a little clearer for you.

 

- Donna G.

[+TheGertridgeExplorers]

Also, don't mix projections and datums together. A datum is a mathematical model of the shape of the earth. NAD27 and NAD83 are datums defined by ellipsoids. Actually NAD27 is Clarke's 1866 ellipsoid tangent to the earth at Meade's Ranch, Kansas and oriented in a specific direction. NAD83 is the GRS80 ellipsoid and is earth-centered.

Projections are the way a point on the earth is transferred to a flat piece of paper. Imagine wrapping a flat piece of paper (map) around a slightly squashed basketball (the datum) in different ways (projection). Cylinderical and Conical are the most common. Then imagine shining a light from the center of the basketball and drawing points on the paper where the points on the basketball are "projected". Now you have a map. So you can have a conical(Lambert) or cylindrical(UTM) projection in NAD27 or NAD83 and the coordinates can be given in feet, metres or lat/long.

Gee, I guess maybe I didn't sleep through all of those boring classes at university! I hope this makes things a little clearer for you.

 

- Donna G.

[JDMC]

Very nice summation, DonnaG.

[+Sea_Dog]

I can't help you with your map questions but I know a little about Danger Close. Currently for 5" Naval Guns, danger close is when friendly forces are within 750 meters of the target. I've been involved with Naval Gunfire Support off and on for 17 years. I took the below picture of a 16" Gun on the USS Iowa. It is capable of delivering a 2700 lb projectile over 20 miles with a great degree of accuracy. Too bad we didn't have GPS inputs to our Fire Control Systems back them. The hardest part of Naval Gunfire Support is figuring out where exactly on the map we are. We can't usually see our targets so spotters give us the coordinates. GPS inputs these days make my job a lot easier and Naval Gunfire Support a whole lot more accurate!

http://img.Groundspeak.com/user/11668_600.jpg

[+Chris n Maria]

quote:

---

Originally posted by Kouros:

How do people deal with OS grid references?

---

You could always use my spreadsheet that converts to & from OS Refs. Mail me or see the link below for more details.

 

Chris

"We're not lost - we just don't know where we are"

London & UK Geocaching Resources: http://www.sheps.clara.net

[Geonavigator]

If the Foreward Observer is located near the the target when he is requesting artillery fire support, "danger close" is a waring to the Fire Direction Center the observer is close to the target. Then FDC will report "Angle T" to the observer Do you remember "Angle T"? Did you serve as a Foreward Observer? Army,or Marines? If so, I'm sure you are an expert in Land Navigation.

[Geonavigator]

The USGS Topographic maps and Maptech software that I am using is NAD-27 map datum. I set my GPS and map software to NAD-27 when I use those maps. When using WGS-84 coordinates on a NAD-27 maps, there will be a constant position error. I set my GPS and map software to WGS-84 when entering WGS-84 coordinates. The GPS and map software will convert WGS-84 coordinates to NAD-27 when the GPS and map software settings are changed to NAD-27. Using this procedure WGS-84 coordinates can be accurately displayed on NAD-27 maps. (I think)