Calculating Lot Size From Assessor's Map

[Victoria Lawrence]

I am a little confused on calculating dimensions listed on an assessor's map. My mentor said to multiply two sides, then the other two, add them together and then divide by two. My query is that the lot resembles a trapezoid with all four sides at different dimensions. I multiplied the front of the lot and the left side, and then the back of the lot and the right side and came up with a particular number. I then decided to multiply the front of the lot and the right side, and then the back and the left side just to see what it would be. It was a different number altogether (about an 800 SF difference). Now I am really confused. Is there a better or easier way to calculate these dimensions so that I am sure I will come up with an accurate lot size? Here is what I did....

lot size = Front (46.2) Right (140.66) Back (114) Left (115.16)
140.66 x 114 = 16035.24
115.16 x 46.2 = 5320.39
16035.24 + 5320.39 divided by 2 = 10,677.81

then

140.66 x 46.2 = 6498.49
114 x 115.16 = 13128.24
13128.24 + 6498.49 divided by 2 = 9,813.36

Any suggestions would be greatly appreciated.

 

[Randy Beigh]

Victoria

We have all gotten used to computers to do this work and I am no math genius, but here is the way I did it.

Add the 2 parallel sides together. 46.2 + 114.00 = 160.20 divide by 2 = 80.1.

140.66 + 115.16 = 255.82 divide by 2 = 127.91.

Multiply 80.1 by 127.91 = 10,246SF.

Also check page D6 of Marshall Swift manual and page 474 of The Dictionary of Real Estate Appraisal, 3rd edition. Page would depend on which edition you have.

 

[Ramon Tate]

In the words of my high school math teacher, may he rest in peace,.... "That dawg won't hunt"

What your mentor suggested won't work. There are many potential sizes for a tract with those dimensions. I played with the dimensions with my deed plotter program a bit and got one plot that calculated to 9579 s.f. and then another that was 9135 s.f. Finally, for fun, I tried to see how much smaller it could be with those dimensions.....I was able to get description closure with a calculated area of only 6493 s.f.

Bottom line, you cannot calculate a size for irregularly shaped properties using only the dimensions of the sides. The angles that they meet determine the square footage that is enclosed.

Doesn't the assessor's property card contain the area calculation performed by the County? Why not use that?

For future reference, if you don't have an area calculation program available on your computer, the most basic way of calculating area is to physically bisect the mapped tract with rectangles and right triangles. You start with the largest rectangle or triangle you can fit within the boundaries and then progressively cover the smaller areas. Claculating area for those is easy. Just use as many as you need to fill up your subject tract and add the area calculations together.

 

[Em Tee]

Yikes! Dr. Math

 

[Ramon Tate]

Originally posted by Mary Adamson@Apr 28 2005, 07:24 PM
Yikes! Dr. Math

Great Link Mary!!!

I'll be adding that link to my kids' computer. THANKS!

 

[Em Tee]

Ramon:

That's how I knew about that site - I've used it for my kids too!!

 

[Stone]

Thanks Mary, that is a good site. I passed it along to several friends, including a teacher.

 

[Jo Ann Meyer Stratton]

And if you have the Apex program--the quickest, easiest and most accurate way is to use their Survey Module.
Click on Help
Click on On Line Manuel
Chapter 6
Pages 141-180

Alt S
Select Call
Type in degrees minutes seconds
Direction
distance
enter
Select Call----etc, etc, etc

Square footage will appear at the bottom of your computer screen, if you print out the sketch, both square footage and acreage will print.

But yes you need to understand the concept of calculating dimensions also.

 

[Rick Kulman]

Victoria

You are being given incorrect information by your mentor. You can't average the opposite sides and then multiply to get the correct square footage. It won't work.

I quickly ran your measurements thru Deed Plotter+ by Greenbrier Graphics
www.greenbriergraphics.com and came up with areas ranging from 7,837 SF to 9,229 SF.

I'm sure if I spend more time adjusting angles, I can get greater or lesser areas.

The 4 skethes I ran through the deed program are in the attached pdf file.

Rick

 

[joyce bachemin]

This web address will help calculate any 4 sided lot. Just type in the dimensions.
Hope this helps.

www.land.net/calc_four.php

John Bachemin