Mal Datums

Hello,

It is important to understand that a so called "Map Datum" like WGS84 has absolutely nothing to do with the update status of a map! Even a map which the last time has been updated as to the map details in 1966 could possibly be referenced to WGS84. Is there a 1966 map datum at all? Have a look at

https://wiki.gis.com/wiki/index.php/Datum_(geodesy)

Regards

reinhardz

Thanks for the reply Reinhardz,

Thanks for the reply Reinhardz,

The paper map in question is Australian Geodetic Datum 1966.

So from what you are saying, a GPS point taken in WGS 84 will not display at the correct UTM  location on an older map ?

Thanks,   Iain

Hello,

Again, I have to point out that displaying the correct location has nothing to do with the map's age or the geodetic datum's age. It's just the question whether the geodetic map datums are identical (location will be shown in the same place) or different (location will be shown at different places).

There are equations to transform positions from one map datum to another one: I am sure that searching the Web will give you sufficient results.

Regards

reinhardz

Thank you.  Clearly My understanding of how various datums relate to a position set in the GPS is lacking.  I will read further,  Iain

I think I should give an example.

Robert A. Pease, unfortunately having passed away much too early, a staff scientist at (former) National Semiconductor and then well known by engineers developping electronic circuits, had regularly written a column in Electronic Design magazine; in his leasure time he had been a passionate hiker. So in his column of October 25, 1994, he wrote about the GPS system, the purchase of one of the first consumer GPS receivers and his experiences with it during a hike that lasted several days. In the report he mentioned that the GPS receiver regularly displayed positions deviating about 100 yards from the real position and blamed that to the then deliberate inaccuracy of the signals intended for civilian use (Selective Availability).

A year and a half later, he wrote an addendum regarding the mentioned position deviations. The precision could be doubled by just activating the correct (geodetic) datum NAD27! Obviously, the default setting had been WGS84. But you can bet that - when hiking - he wasn't using a map editioned in 1927!

I really liked his columns!

reinhardz

Hello again, Yes this exactly my scenario.

Often I will mark a location with the GPS without a paper map.  Then, later on search for the location on a map that may differ in map datum to that used when I marked the point with the GPS.  If I navigate only using the GPS then the selected datum probably makes little difference - you can navigate between points with accuracy.  The issue relates to when you are transferring from GPS to map or from map to GPS.

Obviously (unfortunately) the GPS will not automatically convert a coordinate taken in WGS84 to the datum that is used on the map you are viewing the coordinate.  Therefore, the only way I can see is to convert the coordinates to the datum of the map you ate using.  Likewise, if you are transferring from map to GPS, the GPS datum should be set to the datum of the map.

Appreciate your time in answering what is probably a very basic concept.

Regards,  Iain

Hello,

Isn't the geodetic datum of the maps you are using (obviously Australian Geodetic Datum 1966) included in 66ST's list of available map datums which can be selected? With my 64sx I could choose Australian Geodetic Datum 1966 or Australian Geodetic Datum 1984:

64sx: Main Menu -> System -> Position Format

Just for clarity: Position format (upmost frame) has nothing to do with map datum and is selectable independent of the chosen map datum!

Having activated this setting your problem with incorrect displayed location should be solved.

Regards

reinhardz

To transfer a point from a paper map to a handheld GPS, read the coordinates from the map. Note the datum shown on the map. Set the GPSr to the specified datum, and if necessary to the proper coordinate format). Manually input the coordinates you read off the map into the unit (as a waypoint). The important thing here is that the GPSr datum and the map datum MATCH at the time you enter the coordinates.

If you want to continue to reference the paper map and the GPSr simultaneously, leave the GPSr datum set to the map datum. If not, set the GPSr back to the default WGS84 datum. The GPSr will automatically “convert” among datums (and coordinate systems).

If the unit does not support the datum and coordinate system used on the map, you will have to convert manually, perhaps with the help of an online site.