Oregon 550: Setup/Position Format and Map Datum/Map Spheroid

[+RickandWyatt]

Hi:

 

Apologies if this has been asked before... I searched but came up empty (because "map" and "datum" aren't longer than five letters. Darned if I could figure out how else to )

 

When going through the various options in "Setup", I came across "Position Format." I chose that, and was presented with three different sub-menus: "Position Format", which I understood, and "Map Datum" and "Map Spheroid", neither of which I understood.

 

Both are currently set for "WGS 84", but I have no idea if that's correct or not.

 

Can anybody help edumacate me on these esoteric options? I'm currently leaning towards choosing "Wake-Eniwetok" just because I like atolls, but I'm guessing that would be sub-optimal.

 

Thanks!

 

RickandWyatt

[+dfx]

for all geocaching purposes: h dd mm.mmm and wgs84/wgs84.

[mtbikernate]

It depends on the map you're using. The data on this website assumes WGS 84 datum (there's a spheroid with the same name), but if you're working with an old paper topo, you'll switch your datum to whatever the map uses - NAD27 or NAD83.

 

Essentially, the Spheroid is a model that represents the shape of the Earth. The planet is not perfectly spherical. It's shorter pole-to-pole than it is across the equator, and the southern hemisphere is slightly bulkier. It's kinda like a sphere, hence the term "spheroid". The model gets updated from time to time because tectonic plates shift, magnetic poles shift, and the distance from the moon changes...so the shape of things deforms.

 

Datum is the set of measurements that determine sea level, peak elevations, and stuff like that that are a bit more local in scale than what the spheroid reflects. Both spheroid and datum have dates associated with them, because the time they were measured is quite relevant.

 

A map can be projected (3D surface projected onto a 2D plane) using many different methods, but all with the same spheroid and datum, depending on the exact location is your area of focus and whether you want to preserve areas or shapes.

[+RickandWyatt]

Most excellent, thanks!

 

Blind squirrels, acorns, etc.

 

Thanks very much for helping me to learn one more thing I don't have to think about again.

 

All the best,

 

Rick

 

for all geocaching purposes: h dd mm.mmm and wgs84/wgs84.

[+RickandWyatt]

Thanks very much for your detailed reply.

 

I had already understood everything after the first paragraph; it's the first paragraph that I needed to know but didn't.

 

Thanks again for the quick and thorough answer. Much appreciated.

 

Rick

 

It depends on the map you're using. The data on this website assumes WGS 84 datum (there's a spheroid with the same name), but if you're working with an old paper topo, you'll switch your datum to whatever the map uses - NAD27 or NAD83.

 

Essentially, the Spheroid is a model that represents the shape of the Earth. The planet is not perfectly spherical. It's shorter pole-to-pole than it is across the equator, and the southern hemisphere is slightly bulkier. It's kinda like a sphere, hence the term "spheroid". The model gets updated from time to time because tectonic plates shift, magnetic poles shift, and the distance from the moon changes...so the shape of things deforms.

 

Datum is the set of measurements that determine sea level, peak elevations, and stuff like that that are a bit more local in scale than what the spheroid reflects. Both spheroid and datum have dates associated with them, because the time they were measured is quite relevant.

 

A map can be projected (3D surface projected onto a 2D plane) using many different methods, but all with the same spheroid and datum, depending on the exact location is your area of focus and whether you want to preserve areas or shapes.