Calculate polygon area Cartesianarea() or Area()

[esrispy]

What is the ddifference between these two area calculations in
MapInfo?
I find that the methods give slightly different result, which one is
recomended to use If I want to calcultae lot-area for example?

MapInfoarea = CartesianArea(Selection.obj , "sq m")

MapInfoarea = Area(Selection.obj , "sq m")

[Spencer Simpson]

Use Area() unless you're taking a big performance hit somewhere.

There are three area functions:

SphericalArea() measures areas on the surface of the Earth, using spherical
trigonometry calculations. It's theoretically more accurate, but is more
complicated and takes more time to calculate.

CartesianArea() measures areas of objects as if they were drawn on a flat
plane, using analytic geometry on coordinates referenced to table's
coordinate system, if that table is in a projected coordinate system. This
theoretically takes less time to calculate, but it's less accurate, and if
the table is in a lat/long coordinate system, it returns an invalid result.

If your study area is small, the difference between the two should be
negligible.

Area() abstracts the two so you don't have to decide: it uses
CartesianArea() if the data is in a Nonearth or Layout coordinate system,
and SphericalArea() if it's in an Earth coordinate system.

For each given project that you want to use this function in, you want to
select one or the other, based on the project's requirements, and use it
consistently.

ATTN MAPINFO:

It's a bit weird that CartesianArea() skips the "current coordinate system"
and goes right to the table's coordinate system. If you've set the current
coordinate system to some projected coordinate system, you should be allowed
to extract areas using CartesianArea(), even if the table is in lat/long.

HTH
________________________________

Spencer

[esrispy]

Thanks! that made sense.
We just changed to a new coordinate system and just saw the difference
after this.
Before the datum shift we had everything set to "non earth" so I guess
it did Cartesianarea for both methods before..

Chers//Martin

[Spencer Simpson]

That's more than just a datum shift. You're in a whole new universe now ;)

________________________________

Spencer

-----Original Message-----
From: mapi...@googlegroups.com [mailto:mapi...@googlegroups.com] On
Behalf Of esrispy
Sent: Tuesday, April 21, 2009 2:46 AM
To: MapInfo-L

[Tekraj Chhetri]

Hi,

Could you provide a reference to literature that describes area calculation in detail including formula?

Thanks.