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Permalink Reply by Mac Davis on May 21, 2009 at 20:37

I don't think that works.

Consider a rectangle 100x1 feet.
Sum the four sides/divide by 4 square the results. (202/4)**2 = 2550.25 sq ft.
Actual area = 100 sq ft.

Yes, it's an extreme example, but it illustrates the error of the method.

Consider a less extreme example, a rectangle of 30 x 100 Feet.
260/4**2=4225
Actual area=3000
The error is 40%

That method sort of assumes the lot is a square. The less like a square is, the less accurate it is.

I've also heard the idea of averaging the opposite sides and calculating it like a rectangle.

i.e. ((A+C)/2)*((B+D)/2)

This guestimate is more interesting to me, but it fails if the angles at the corners are not close to right angles. It's probably a pretty good estimate for lots where the lot is pretty close to a rectangle, which DOES describe a pretty large percentage of the lots.

However, consider an isosceles trapezoid shaped lot. This formula will be off by a factor equal to the sine of the angle at any of the corners.

While most lots may be approximately rectangular, I suspect this is a lot at the end of a cul-de-sac, where the formula performs most poorly.

I think these guestimates will normally overestimate the size of a lot. The conspiracy theorist in me tends to think that real estate agents and tax assessors have an incentive to overestimate the size, so they use these formulas. (Only slightly joking.)

Note that even Brahmagupta's formula calculates a maximum possible size for the lengths given, not necessarily the real size.

By the way, the "average opposite sides" formula gives 0.2516 acre for the 146' + 127' + 121' + 39' lot , which is 6 percent higher than Brahmagupta's formula, even for such an oddly shaped lot.

Despite the long name, Brahmagupta's forumula is pretty easy to program into a spreadsheet. It's probably worth the effort for anyone who deals with this on a regular basis. Just remember, it's still a calculation of the maximum possible area, not necessarily the correct area. It's better than the other rules of thumb, though.

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Permalink Reply by The_Baron on May 21, 2009 at 8:20

Is this discussion for real?

Are we really still discussing this?

Post a plat and we will know how big the lot is.

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Permalink Reply by Mac Davis on May 21, 2009 at 8:37

- Is this discussion for real?

yes

- Are we really still discussing this?

yes

It seems to me that the question of how to calculate the possible range of acreage given only the sides of the lots is extremely relevant. A lot of times, people have the lengths of the sites, but no other info.

Unless I've screwed up somewhere, we now know how to calculate this.

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Permalink Reply by The_Baron on May 21, 2009 at 9:04

Carry you butt down to the assessors office, and look at the plat. Using the plat you can calculate the exact acreage. All this mathmatical theory is a waste of time.

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Permalink Reply by The_Baron on May 21, 2009 at 10:15

So will trying to learn the ins and outs of the assessors office.

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Permalink Reply by Mac Davis on May 21, 2009 at 19:56

Let's see,

Get in the car, drive 20 miles or so round trip to the assessor's office. (or further if you don't live close to the assessor's office.
Figure out how to get a copy of the plat.
Then go home and work out the math.

or

Put a little thought into it and figure out what it might be from the information you already have.

Which one of these sounds like a bigger waste of time.

By the way, how do you get a copy of the plat? What does it cost? How much time does it take? (Not being a smart ass, I'd really like to know, because it IS a good idea even if you DO know the area.)

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Permalink Reply by The_Baron on May 22, 2009 at 3:22

The driving and time aside, I personally believe every landowner should have a copy of the assessors plat for land they own. You can make your trip even more worthwhile by checking to see what they have assesed you for. You can challange your assessment if you feel its too high and many times they will knock a little off just to get you out of the office. Come with a list of reasons of why you should be assessed lower, leaky plumbing, old roof, old water heater, etc....

I will speak of the caddo assessor, but most are set up in a simmular manner.

To find your plat, you need to know if you are inside the city (shreveport) or out.

Depending on which one you are will determine what set of books you look in. The staff will show which ones are which. The books are by Township-Range-Section-Subdivision

For example, If you have land in section 30-T16N-13W, you are in the book that includes tract 161330.

Find the main page for your section. This will show all tracts or subdivisions in your section.

The charge for the section plat varies depending on what size you want (letter to 11x17), but is only a dollar or two.

If your land is in a subdivision, there will be a seperate page for that subdivision. The subdivision page will have the lots drawn out and maybe even have the surveyors bearings and acreage. In any case the plats are fairly presise. The same costs apply for copies.

If you get confused or can't find it, ask for help, thats why the staff is there, to help the general public.

Depending on your know how, it should only take a few minutes to find it. The staff makes the copies, usually on the spot, but it depends on how crowed it is. While your there, get the assessment info on your tract, only $0.50

If you know your s-t-r you are in good shape, if not you will need to ask, the staff can look you up by address or name.

They have computers for the general public, but they are a pain to use.

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Permalink Reply by BirdDawg, plain ole' idiot on May 22, 2009 at 0:08

Mac,
What you are coming up with is an estimation.

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Permalink Reply by RAY on May 22, 2009 at 1:39

the reason I asked for help was because the assessor's office had my lot at .254 acre........ but the landman came up with .198 acre..... either I have a 1/4 acre or a 1/5 acre.

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Permalink Reply by BirdDawg, plain ole' idiot on May 22, 2009 at 2:35

Ray,
If you could provide me a copy of a subdivision plat (your lot) or a legal description with distances AND bearings, I could compute it for you exactly. No guesswork then.

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Permalink Reply by Mac Davis on May 22, 2009 at 23:04

0.198 acre is a very suspicious number. That is the area of a triangle with sides of 147x161x121. (161 equals 128+39) That is the minimum acreage your lot can be given your dimensions. (e.g. the angle between the 128 and 139 sides is exactly 180 degrees.

It sounds to me like the landman is using an estimation method that deliberately gives the smallest possible area.

It sounds to me like he's trying to cheat you. i.e. it's not a wild ass guess, it's the exact same minimum number I got to 3 decimal places for the minimum area.

Where does your 146' + 127' + 121' + 39' measurement come from? Is it possible the assessor has some slightly different measurements? .254 is 7 percent higher than the possible size of those dimensions. However, that difference is probably not going to make any difference in your assessed value.

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Permalink Reply by Mac Davis on May 22, 2009 at 22:42

No, what I'm coming up with is the MAXIMUM POSSIBLE acreage for a piece of land with those dimensions.

Unless I've miscalculated, his plot cannot be more than 0.2382 acre.

I would be happy if someone else would look up Brahmagupta's formula and confirm my conclusion or dispute it.

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