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pedroantonio

• Guest
Adjust the area of a polygon
« on: March 08, 2014, 02:43:49 PM »
Good evening. I am looking for a lisp file that fixes the size polygon.
I designed the polygon knowing the lengths of sides and its area. But  at the end the area is not the same.
Look the attach *.dwg
Thanks

roy_043

• Water Moccasin
• Posts: 1876
Re: Adjust the area of a polygon
« Reply #1 on: March 09, 2014, 03:44:48 AM »
If I look at the green rectangle in your drawing I notice that the long sides are not exactly parallel and one is 165.90 long and the other is 165.93. Both short sides are 30.00. The conclusion must be that this is not an exact rectangle. The question is then: Why is 4977.00 the correct area?

Don't you typically need measurements for multiple diagonals to accurately draw these land parcels?

But in theory you can create a lisp that scales the polyline using this factor:
Code: [Select]
`(sqrt (/ expectedArea currentArea)).`

pedroantonio

• Guest
Re: Adjust the area of a polygon
« Reply #2 on: March 09, 2014, 04:19:08 AM »
the dimensions and the area of ​​the polygon I get from PLANNING.
Quote
But in theory you can create a lisp that scales the polyline using this factor:
Code: [Select]
(sqrt (/ expectedArea currentArea)).

Yes but if i scale the polygon the dimensions will be change.I don't want to change the dimensions.I need the polygon to have the dimension I give and the area I give.
For example a lisp
1) Select a close polyline
2) Give the area of the polygon
Then adjust the polygon (Keep the dimensions)
Something like mapcheck but with specific area
« Last Edit: March 09, 2014, 04:25:05 AM by Topographer »

snownut2

• Swamp Rat
• Posts: 971
Re: Adjust the area of a polygon
« Reply #3 on: March 09, 2014, 10:04:13 AM »
pedro,

You are trying to adjust area by changing polygon angles, with no reference angles to begin with, there can be multiple solutions to this....After looking at your drawing, you realize that by doing this you will be affecting the area of any adjoining lot, you will be opening up pandora's box...
« Last Edit: March 09, 2014, 10:10:54 AM by snownut2 »

pedroantonio

• Guest
Re: Adjust the area of a polygon
« Reply #4 on: March 09, 2014, 12:35:06 PM »
Yes snownut2 you are right but i was thinking a lisp

1) Select a close polyline
2) Give the area of the polygon
3)Select the point  (one or two points to change the angle)

Then adjust the polygon (Keep the dimensions)
Something like mapcheck but with specific area

What do you think ? I think this way is better .

snownut2

• Swamp Rat
• Posts: 971
Re: Adjust the area of a polygon
« Reply #5 on: March 09, 2014, 12:40:50 PM »
Pedro,

Think about that, you need to adjust every single angle to maintain the line lengths, not just one or two.

Bruce

pedroantonio

• Guest
Re: Adjust the area of a polygon
« Reply #6 on: March 09, 2014, 12:47:50 PM »
Not exactly . If i want to change point 2 ,i need to select point 1 ,point2,point3
look the dwg

snownut2

• Swamp Rat
• Posts: 971
Re: Adjust the area of a polygon
« Reply #7 on: March 09, 2014, 01:22:12 PM »
Maybe I was a little dramatic when I said every single angle, however I still stand that you need to adjust multiple (many) angles to maintain the line lengths.  (on lots with only a few lot lines it will be all angles).

In your sample drawing, the angle prior to pt1 along with pt1, pt2 & pt3 angles would all be affected by making any area change.

Affecting 4 out of 6 angles, and affecting the area of the abutting lot to the rear.

mailmaverick

• Bull Frog
• Posts: 470
Re: Adjust the area of a polygon
« Reply #8 on: March 09, 2014, 01:35:44 PM »
Dear Friend,

I did some research and found that there are multiple solutions to your problem.

There can be multiple combinations of 6 sided polyons with length of sides given by you and area also as given by you.

As an example, I am attaching DWG file in which I have made three 6 sides polygons with each side as given by you and area also as given by you.

You can see that apart from these three, there are even more multiple solutions.

pedroantonio

• Guest
Re: Adjust the area of a polygon
« Reply #9 on: March 09, 2014, 01:41:42 PM »
yes but if i chose only  one angle to be change  like point1,point2,point3 not all the angles , it will change only the angle in the point 2

roy_043

• Water Moccasin
• Posts: 1876
Re: Adjust the area of a polygon
« Reply #10 on: March 09, 2014, 02:02:06 PM »
Isn't this really a case of rounding?
2517.12 ("wrong"); Rounded: 2517 ("correct").
4977.33 ("wrong"); Rounded: 4977 ("correct").

pedroantonio

• Guest
Re: Adjust the area of a polygon
« Reply #11 on: March 09, 2014, 02:04:15 PM »
yes but in same othe option you will have a bigger difference for example

2517.12 ---->2517.90 or 2519.30 etc

snownut2

• Swamp Rat
• Posts: 971
Re: Adjust the area of a polygon
« Reply #12 on: March 09, 2014, 02:06:57 PM »
Pedro,

Why don't you try drawing a new lot configuration with a new area by adjusting the angle @ pt2 ONLY, keeping all line lengths are they are then post result here, that way we can understand exactly what it is you are after.

If as in Roy's post it is caused by rounding, instead of using an "=" sign in the comparison statement use  "equal" with a fuzz factor.

Code - Auto/Visual Lisp: [Select]
1. (if (equal area1 area2 0.1)
2.    (carry on)
3. )
4.

Bruce

roy_043

• Water Moccasin
• Posts: 1876
`(setq differencePercent (* (/ (- 2517.90 2517.0) 2517.0) 100.0)) ; => 0.0357569 percent`What are we talking about. Aren't these tolerances normal in topography?