Need to calculate the area of your land?

…Applying fertilizer or herbicide, sowing seed, etc.

Easy if your field is rectangular but fields tend not to be true regular shapes. Therefore, break the field into triangles, apply the following algebra, then sum the triangle areas to obtain the true area of the whole field. You can use Google Earth or www.PointAsia.com (generally much better resolution for Thailand) to see and measure your land (contains a linear measurement tool).

Algebra:

√ ( ((A+B+C) / 2) x (((A+B+C) / 2) – A) x (((A+B+C) / 2) – x (((A+B+C) / 2) – C) )

Paste this into cell A4 of a spreadsheet, after inputting the length of each side of the triangle in cells A1 to A3:

=SQRT(((A1+A2+A3)/2)*(((A1+A2+A3)/2)-A1)*(((A1+A2+A3)/2)-A2)*(((A1+A2+A3)/2)-A3))

To convert square meters to rai, divide by 1,600.

Rgds

Khonwan

Never could handle algebra,I just add the sum of two adjoining sides and divide by 2 ,same for the other sides then multiply the two numbers.

Never could handle algebra,I just add the sum of two adjoining sides and divide by 2 ,same for the other sides then multiply the two numbers.

Hi Ozzy

Your method of dividing by two the factor of two sides (opposite & adjacent) only works for right-angled triangles. Doesn’t even come close for irregular triangles. The method I’ve posted works for regular and irregular triangles.

Fortunately, no need to understand algebra – just past the formula into your spreadsheet.

Let me provide an example: I’ll choose the irregular-shaped field that my house sits on. It has six sides: 123m, 77m, 198m, 39m, 44m, 138m.

I’ve broken the shape into three triangles measuring 123m, 77m, 164m, plus 190m, 198m, 165m, plus 39m, 44m, 52m. The correct area calculates out at 4,505m² + 14,469m² + 837m² = 19,811m² = 12.382 rai.

Rgds

Khonwan

Most true, long time farmers can look at a piece of land, step off 1 side and give a pertty accurate guesstaimate to acres involved. Never saw a cowman who could not give wieight of cattle by looking at them in the pasture, normally within 5 % of true wieght. Khonwan's method is a good one for accurate work.

walk round the perimeter with a GPS?

walk round the perimeter with a GPS?

Great, if you have a GPS that has the area calculation function. I purchased my Garmin unit 8 years ago (rarely use it) but, AFAIK, it does't have this facility.

Rgds

Khonwan

walk round the perimeter with a GPS?

Great, if you have a GPS that has the area calculation function. I purchased my Garmin unit 8 years ago (rarely use it) but, AFAIK, it does't have this facility.

Rgds

Khonwan

reread my post, sounded a bit sarcastic. - wasn't taking a dig just making a suggestion. You're right, IF you have a GPS.

walk round the perimeter with a GPS?

Great, if you have a GPS that has the area calculation function. I purchased my Garmin unit 8 years ago (rarely use it) but, AFAIK, it does't have this facility.

Rgds

Khonwan

reread my post, sounded a bit sarcastic. - wasn't taking a dig just making a suggestion. You're right, IF you have a GPS.

Sorry dsys, you've misinterpreted me. I didn't perceive your post as sarcastic in the slightest. I just wish I had a GPS with the facility!

Kind regards

Khonwan

walk round the perimeter with a GPS?

Great, if you have a GPS that has the area calculation function. I purchased my Garmin unit 8 years ago (rarely use it) but, AFAIK, it does't have this facility.

Rgds

Khonwan

reread my post, sounded a bit sarcastic. - wasn't taking a dig just making a suggestion. You're right, IF you have a GPS.

Sorry dsys, you've misinterpreted me. I didn't perceive your post as sarcastic in the slightest. I just wish I had a GPS with the facility!

Kind regards

Khonwan

Garmin e-trex vista works good. If any body is intersted.

Need to calculate the area of your land?

…Applying fertilizer or herbicide, sowing seed, etc.

Easy if your field is rectangular but fields tend not to be true regular shapes. Therefore, break the field into triangles, apply the following algebra, then sum the triangle areas to obtain the true area of the whole field. You can use Google Earth or www.PointAsia.com (generally much better resolution for Thailand) to see and measure your land (contains a linear measurement tool).

Algebra:

√ ( ((A+B+C) / 2) x (((A+B+C) / 2) – A) x (((A+B+C) / 2) – x (((A+B+C) / 2) – C) )

Paste this into cell A4 of a spreadsheet, after inputting the length of each side of the triangle in cells A1 to A3:

=SQRT(((A1+A2+A3)/2)*(((A1+A2+A3)/2)-A1)*(((A1+A2+A3)/2)-A2)*(((A1+A2+A3)/2)-A3))

To convert square meters to rai, divide by 1,600.

Rgds

Khonwan

Nice tool KW. Point Asia has an "Area" function but can't figure out how to use the danm thing. The "linear" measurements are straight forward.

Nice tool KW. Point Asia has an "Area" function but can't figure out how to use the danm thing. The "linear" measurements are straight forward.

Thanks Somtham - I had no idea PointAsia has an area function. I'll check that out.

Rgds

Khonwan

Nice tool KW. Point Asia has an "Area" function but can't figure out how to use the danm thing. The "linear" measurements are straight forward.

Thanks Somtham - I had no idea PointAsia has an area function. I'll check that out.

Rgds

Khonwan

PointAsia area function:

- Right click on map to bring up menu

- Click "Add Measure"

- Click on points of area to be measured

- Double-click on last point (completing the area)

This returns the area in m­² and in rai.

PointAsia area function:

- Right click on map to bring up menu

- Click "Add Measure"

- Click on points of area to be measured

- Double-click on last point (completing the area)

This returns the area in m­² and in rai.

Thank you. Works great.

Theres a wonderful method I learned way back in high school and it was so useful I simply could never forget it no matter how hard I tried.

Works for any polygon no matter the shape.

First, get a list of cartesian coordinates for every vertex on your land. Easiest way is with a GPS, but you can do it with a tape measure if you have to. I've even just used paces to get a quick and dirty estimate at times. Just pick one corner to start, and represent every other coordinate as a horizontal and vertical offset from that point.

Now you should have a list of (Xi,Yi) coordinates. Calculate sigma [ X(i) Y(i+1) - X(i+1) Y(i) ] where sigma represents summation. Divide that number by 2, and that is the area inscribed by your coordinates.

E.g., assume you have a square that is 1 rai. The first coordinate is (0,0). The second is (0,40). The third is (40,40). The final one is (40,0)

Calculate XiY(i+1) - X(i+1)Yi

0: 0*40 - 0*0 = 0

1: 0*40 - 40*40 = -1600

2: 40*0 - 40*40 = -1600

3: 40*0 - 0*0 = 0 (Wrap around to the first coordinate here)

Answer is -3200/2 sq. meters or -1600 sq. meters. (1 rai)

Answer would be positive had I processed the coordinates in the other direction.

Have done some very irregular shapes this way. Works very well.

I was playing with my eTrex Legend GPS and found that it too has an area calculator. It works from what they call tracks. You have to click all around the perimeter it shows you, then it is supposed to give the area in square meters. I never tried it and would most likely have to study on it for some time, but it does have that function.