Map Datum - which one?

[BEESnBUSH]

Hello All,

 

My first post here as a geocacher and straight in at the deep end!

 

I've recently been looking into different Map Datums that my Garmin 60CSx has on it, and it appears that there are quite alot for the rest of the world and what I think is only three for UK. I'm probably wrong on this as I'm new to it, but the ones I found are WGS84, WGS72 and Ord Srvy GB.

 

I was just wondering if there was any way of updating these Datums to newer perhaps more accurate ones, like the OSGB36.

 

As I've said I'm new to this sort of information so any help would be greatfully received!

 

Cheers

[+Viajero Perdido]

All caches on geocaching.com are supposed to be recorded with WGS84 datum, worldwide. Notice the very fine print at the bottom of any cache page.

 

So just set your unit to WGS84 and never touch it again.

 

Using the wrong datum can lead to not finding any caches, or hiding unfindable ones.

[+ecanderson]

If you are using coordinates provided by geocaching.com, your ONLY option is to use WGS84. Using a different datum will cause the coordinates to be interpreted differently, putting you in the wrong location by a few feet or many hundred! Stick with WGS84!

[BEESnBUSH]

Ah I see (or didn't!). Will change it over.

 

Thankyou

[+Bear and Ragged]

Worth a read > http://www.follow-the-arrow.co.uk/resources/gps.html

Written by one of the UK reviewers.

[+DazeDnFamily]

I may not be completely correct, but I think that the GPS system is based in WGS84. So, the default for anything involving GPS should be WGS84. Your GPSr can convert a coord from what ever datum to WGS84 on the fly, however, assuming I'm remembering correctly. The only time I could see you needing another datum is if you have a locally produced map (or map data), of a generally small area, that has an odd datum, or if it was produced before WGS84 (incidentally, 1984).

 

Later!

[Grasscatcher]

Good grief!, no wonder there is so much confusion about datums.....5 or 6 answers / opinions and most are partially correct but none tell the whole story or "why".

 

See if this helps (or hurts)

 

Internally, all GPS's "think" and calculate in WGS 84.

 

What the user defined "setting" on the unit is, defines only how the user wants to "see" the coordinates. ie (for display purposes only) However..... for MANUAL coordinate input, the user must have the unit set(during data input only) on what ever datum the source information is already in.

Example: Set unit on Datum X for manual input of data in X Datum format, and change / set unit on Datum Y for manual input of data in Datum Y format.

 

Now, once the data has been correctly input (now is "internal"), the user can change the unit Datum setting to "whatever" Datum and that is what is "displayed". Conversions are correctly done internally.

 

If you are going to take data from your GPS and plot it on a paper map, then you must set the GPS to whatever Datum the map was created in THEN read/take the coordinates and plot them on the paper map.

Same thing applies in reverse, taking coordinates from a paper map to put into a GPS, set the GPS to the map datum BEFORE inputing the coordinates determined from the map.

 

Good Rule of Thumb.......Just always be aware(and check)when you are going to input data....... GPS needs to be "set" on same Datum as "source" data.

 

Examples: Data from GC.com always = WGS 84, manual data to/ from any maps=(whatever)the map datum.

[+DazeDnFamily]

Good grief!, no wonder there is so much confusion about datums.....5 or 6 answers / opinions and most are partially correct but none tell the whole story or "why".

 

See if this helps (or hurts)

 

Internally, all GPS's "think" and calculate in WGS 84.

 

What the user defined "setting" on the unit is, defines only how the user wants to "see" the coordinates. ie (for display purposes only) However..... for MANUAL coordinate input, the user must have the unit set(during data input only) on what ever datum the source information is already in.

Example: Set unit on Datum X for manual input of data in X Datum format, and change / set unit on Datum Y for manual input of data in Datum Y format.

 

Now, once the data has been correctly input (now is "internal"), the user can change the unit Datum setting to "whatever" Datum and that is what is "displayed". Conversions are correctly done internally.

 

If you are going to take data from your GPS and plot it on a paper map, then you must set the GPS to whatever Datum the map was created in THEN read/take the coordinates and plot them on the paper map.

Same thing applies in reverse, taking coordinates from a paper map to put into a GPS, set the GPS to the map datum BEFORE inputing the coordinates determined from the map.

 

Good Rule of Thumb.......Just always be aware(and check)when you are going to input data....... GPS needs to be "set" on same Datum as "source" data.

 

Examples: Data from GC.com always = WGS 84, manual data to/ from any maps=(whatever)the map datum.

 

So.....

 

Exactly what I said? :laughing:

[+Viajero Perdido]

One sentence:

 

Set your unit to WGS84 and never touch it again.

[+Bear and Ragged]

Not as simple as that ...

 

Unless all you do is use your GPS for geocaching

... And you don't do 'Devious' caches which require you to change the Datum/Lat and Long settings to find the cache...

[xyzee]

You can see both the Lat/Lon and the BNG for your location, on any page that will let you select 2 Wide Data Fields, like at the bottom of the Trip Computer Page. When you Change Data Fields, select Location (lat/lon) for one, and Location (selected) for the other. Then to Menu>Setup>Units>Position Format and select British Grid, which auto changes the Map Datum to Ord Srvy GB. You might want to change the Distance/Speed to Metric if mtrs work better for the BNG. Putting these two Locations in the Trip Computer page Big Numbers, is a lot easier for me to read.

Yaxley shows up as: N 52 19.357 TM 12146

E 001 06.695 BNG 73954

And then there is UTM UPS to play with.

Have fun.

 

The numbers didn't line up right, and I can't edit it, so "read it right"

Edited October 29, 2011 by xyzee

[Grasscatcher]

DazDnFamily,

No, not what you said......The devil is in the details that you DIDN'T say. The what,where, why...

 

....that's what made it longer.

[TheArabianHunter]

Just an observation if your map that is inside your gps is in wgs 84, changing the datum will not change anything relative to coordinates that were taken in wgs84 datum . The map datum is always wgs 84 for Garmin maps and birds eye as well.

[+ecanderson]

DazDnFamily,

No, not what you said......The devil is in the details that you DIDN'T say. The what,where, why...

 

....that's what made it longer.

It could have been made longer yet for an even fuller explanation .. but don't forget, the OP states right up front "My first post here as a geocacher and straight in at the deep end!" For that purpose, the "use WGS84 or the coordinates from gc.com will be misinterpreted" is more than adequate.

[Grasscatcher]

DazDnFamily,

No, not what you said......The devil is in the details that you DIDN'T say. The what,where, why...

 

....that's what made it longer.

It could have been made longer yet for an even fuller explanation .. but don't forget, the OP states right up front "My first post here as a geocacher and straight in at the deep end!" For that purpose, the "use WGS84 or the coordinates from gc.com will be misinterpreted" is more than adequate.

Yes, until you consider that several of the follow up posts implied that there were no conditions that anything other than WGS 84 should ever be used.

No since in whupping a dead horse.....enough said....I won't try to help in the future.

Edited October 29, 2011 by Grasscatcher

[+ecanderson]

.I won't try to help in the future.

Sure you will. As will I. We both know that...

 

Hope you still have some trees over there. It's a mess up in the metro area.