In the US, real property distances and areas are calculated with Cartesian
math (ie, right angle trigonometry) on a plane at an average surface
elevation. I live about 6000 feet above sea level, so the area of a parcel
in my neighborhood is about 0.03% larger than it is when projected onto the
ellipsoid (sea level). We would no sooner use spherical areas or great
circle distances than we would use slope distances. By default, MapInfo
calculates spherical areas and great circle distances, which used to bug
me, and I used to waste a lot of time "working around" this "problem"
despite the fact that it is really a more accurate representation of
ellipsoidal earth than Mr. Pythagorus offered us. But with MI 5.5, you can
choose either spherical or Cartesian area and distance calculations. My hat
is off to whoever to the unsung hero at MI is that added this feature.
With regard to Bill's initial query; real property (deeded land, etc.) in
the US is assumed to be "flat" when calculating area and distance. Both in
regard to the slope (topography), and with regard to the surface of the
ellipsoid. Even using Cartesian math will produce minor differences when
calculating distances and areas on a projected coordinate system versus a
"local" coordinate system, because of the above sited difference is surface
elevation, and because of the map scale factor, which is a function of the
distance from the central meridian. Yeah, all this makes a difference at
$30k / acre, or better than a million an acre we see around here, so
explain that to you're your local relator.
Rich
At 10:59 AM 1/20/2000 +1000, you wrote:
>Bill,
>In Australia all boundary distances are converted to a horizontal distance
>on a plane surface giving a lesser area than the actual surface area in the
>case of sloping land.
>
>When and if we go to a digital cadastre, I would imagine that spherical
>distances would be used and projected to a mathematical surface based on the
>new Geodetic Datum of Australia (I could be wrong in this assumption and
>stand to be corrected)
>
>Regards
>Peter Graham
>
>----- Original Message -----
>From: "Bill Thoen" <[EMAIL PROTECTED]>
>To: "MapInfo-L" <[EMAIL PROTECTED]>
>Sent: Thursday, January 20, 2000 9:05 AM
>Subject: MI A Question for Surveyors
>
>
>> Is there a surveyor in the house?
>>
>> I was wondering how property boundary line distances and acreages
>> are calculated in sloping terrain. Are boundary distances
>> reported as the distance along the ground surface or are they
>> recorded as projections of the distance along the ground into the
>> x-y plane? In other words, if a line measured between two
>> monuments is 1200 ft along the ground, and one monumnet is 600 ft
>> higher than the other, is that leg recorded as being 1200 ft or
>> 1039.2 ft (1200*cos(30 degrees))?
>>
>> Given that, is acreage calculated based on surface area or as
>> area projected into the X-Y plane? Seems to me if you have to
>> pay $30,000/acre it would make a difference.
>>
>> - Bill Thoen
>> ----------------------------------------------------------------------
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>
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>
>
Richard W. Greenwood, PLS
Greenwood Mapping, Inc.
[EMAIL PROTECTED]
(307) 733-0203 fax: 733-4018
http://www.GreenwoodMap.com
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