area calculation

Unfortunately, Google Earth Plus does not offer the means to
calculate
surface areas of polygons or even triangles, this is
reserved to the
expensive version Google Earth Pro ($400).
However, you do have the means to measure straight line
distances
(click Tools, Measure) and, I have found the best way to
measure
surface areas accurately is as follows:
1) Print out a close-up image of the land area and mark out
the polygon
to be measured, labelling the points A,B,C...etc.
Alternatively, pin
mark each point on screen.
2) Use the Measure function to measure the distance between
each
combination of the points of the polygon.
3) Subdivide the polygon into triangles ABD, ABD, ACD etc
etc.
4) Use Excel or similar application to make a table which
summarises
the length of each side of every triangle with sides a,b,c
as column
headings.
5) Create a 4th column heading s (=semi-perimeter) and give
each cell
below that heading the formula required to equal (a+b+c)/2.
6) Create a 5th column heading for area and insert the cell
referenced
formula for the area of a triangle [area =
sqrt(s*(s-a)*(s-b)*(s-c))]
7) Add the areas together to find the total surface area of
the polygon


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On 3/20/06, sarge <iansargeantcompuserve.com> wrote:
> 3) Subdivide the polygon into triangles ABD, ABD, ACD
etc etc.
> 4) Use Excel or similar application to make a table
which summarises
> the length of each side of every triangle with sides
a,b,c as column
> headings.

I can't follow you. I tried using a shape with 6 points and
I
couldn't. Do I have to make all posible triangles? Should I
make only
non overlapping ones? A picture will clarify it all I think.
You can
host pictures for free here: http://imageshack.us/
Best regards,
SB.

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Sorry it wasn't completely clear.
All polygons can be sub-divided into a set of contiguous
triangles.
Since there is a formula for measuring the area of a
triangle based
purely on the length of the three sides (angles not
required), it is
possible to calculate the area of a polygon by aggregating
the areas of
the triangles.
You do not need to measure the distance between each pair of
points of
a polygon. For simple polygons, you can identify a central
point that,
when radiated to the other points, gives you the required
set of
contiguous triangles (not overlapping). Measure the sides of
all these
triangles, calculate the area of each and add them all up.
If the
polygon is very complex and does not have a point from which
to
radiate, it may be simpler to divide the total polygon into
more than
one polygon.
I have used the conventional notation for the lengths of
each side of
any triangle as a,b & c. If you decide to aggregate the
areas of the
triangles on a spreadsheet, make the 5 column headings a,b,
c, s (for
semi-perimeter cell-referenced formula) and area as
cell-referenced
formula. Make the row labels the triangles you have
identified for
aggregation (ABC, ACD, ADE etc if A is the radiating point
on the
polygon).
Hope this is clear!


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