This message is in MIME format. Since your mail reader does not understand
this format, some or all of this message may not be legible.
Robert,
There are formulae out there but be prepared to remember your calculus! Try
searching the net for the green formula or the gauss-green formula.
As a surveyor, if I want to calculate an irregular area, I'd use a
planimeter or use a method based on calculating the area of trapezoids:
Assume a base line below your area and use this to divide your area into
trapezoids. You can then deduct one set of trapezoids from another to get
the total area. e.g..
A
B
C
--------------- base line
A1 C1 B1
Find area A,A1,B1,B,A using formula (AA1+BB1)/2*A1B1 - average of heights *
length of base
then deduct area A,A1,B1,B,C,A.
In terms of co-ordinates, the area of a trapezoid can be calculated with
Y1+Y2 * (X1-X2)
2
where X1,Y1 are the co-ordinates of A and X2,Y2 are the co-ordinates of B.
This can be extended to the whole polygon by using the formula:
SUM(i=1 to i=3) Yi+(Yi+1)
--------- * (Xi-(Xi+1))
2
You must also remember to close the polygon by making the last co-ordinates
in the formula the same as the first i.e. Xn+1 = X1 and Yn+1=Y1.
I hope this makes sense, its difficult to describe without diagrams. You
should be able to find a better explanation off the net.
Andrew Young
Durham City Council
-----Original Message-----
From: Robert Crossley [mailto:[EMAIL PROTECTED]]
Sent: 12 March 2002 06:41
To: MapInfo List
Subject: MI-L Area Calculations
Can anyone point me in the direction of a formula or technique to calculate
an area of irregular shapes (no holes), from a
list of coordinates - without a GIS? The answer can be in square coordinate
units. The formulae uncovered so far
indicate a rather manual subdivision of the shape into triangles, but I
suspect that this is not the only answer.
I know that this is not a Mapinfo question, but it is a geographic question.
Thanks
Rob.
----------------------------
Robert Crossley
Robert Crossley and Associates
9 Short Street
NEW BRIGHTON NSW Australia 2384
P: 61-2-66801309
F: 61-7-????????
M:0419718642
GPS: ?????
----------------------------
_______________________________________________________________________
List hosting provided by Directions Magazine | www.directionsmag.com |
To unsubscribe, send e-mail to [EMAIL PROTECTED] and
put "unsubscribe MapInfo-L" in the message body.
IMPORTANT NOTICE: This e-mail is confidential. It must not be read,
copies disclosed or used by any person other than the main recipient.
Unauthorised use, disclosure or copying is strictly prohibited and may
be unlawful. If you have received this e-mail in error, please contact the
sender immediately.
_______________________________________________________________________
List hosting provided by Directions Magazine | www.directionsmag.com |
To unsubscribe, send e-mail to [EMAIL PROTECTED] and
put "unsubscribe MapInfo-L" in the message body.
