Android Question area of polygon

RB Smissaert

Well-Known Member
Licensed User
Needed a way to get the centre of a polygon, defined by a list of osmdroid_geopoints.
Part of this algorithm is to get the area of the polygon. All this works fine and I get the
centre of the polygon (defined as making the polygon balance on a pin if put at the centre)
Having this area I would like to know how to get the real are in square km or whatever unit.
The actual obtained area value is very small, so there need to be some multiplication factor, but
not sure how to get this factor.

B4X:
Sub GetPolygonArea(lstPoints As List) As Double

    Dim i As Int
    Dim dArea As Double

    'Get the areas.
    For i = 0 To lstPoints.Size - 2
  Dim oGeoPoint1 As OSMDroid_GeoPoint = lstPoints.Get(i)
  Dim oGeoPoint2 As OSMDroid_GeoPoint = lstPoints.Get(i + 1)
        dArea = dArea + _
                (oGeoPoint2.Longitude - oGeoPoint1.Longitude) * _
                (oGeoPoint2.Latitude + oGeoPoint1.Latitude) / 2
 Next

 Return Abs(dArea)

End Sub

Sub FindCentroid(lstPoints As List) As OSMDroid_GeoPoint

    Dim i As Long
    Dim second_factor As Double
    Dim polygon_area As Double
    Dim X As Double
    Dim Y As Double
 Dim oGeoPointCenter As OSMDroid_GeoPoint

    For i = 0 To lstPoints.Size - 2
  Dim oGeoPoint1 As OSMDroid_GeoPoint = lstPoints.Get(i)
  Dim oGeoPoint2 As OSMDroid_GeoPoint = lstPoints.Get(i + 1)
        second_factor = _
        oGeoPoint1.Longitude * oGeoPoint2.Latitude - _
                        oGeoPoint2.Longitude * oGeoPoint1.Latitude
        X = X + (oGeoPoint1.Longitude + oGeoPoint2.Longitude) * _
            second_factor
        Y = Y + (oGeoPoint1.Latitude + oGeoPoint2.Latitude) * _
            second_factor
    Next

    'Divide by 6 times the polygon's area.
    polygon_area = GetPolygonArea(lstPoints)
 General.RunLog("FindCentroid, polygon_area: " & polygon_area)
 
    X = X / 6 / polygon_area
    Y = Y / 6 / polygon_area
 

    'If the values are negative, the polygon is
    'oriented counterclockwise. Reverse the signs.
 '(this works for me, but maybe wrong for other locations)
 If Y < 0 Then
  X = -X
  Y = -Y
 End If
 
 oGeoPointCenter.Initialize(Y, X)

 Return oGeoPointCenter

End Sub

RBS
 

klaus

Expert
Licensed User
Routine to calculate the area in squaremeters.

B4X:
Sub GetPolygonArea(lstPoints As List) As Double
    Dim i As Int
    Dim Lat1, Lat2, Lng1, Lng2 As Double
    Dim dArea, LatScale, LngScale As Double

    'Get the areas.
    Dim oGeoPoint1 As OSMDroid_GeoPoint = lstPoints.Get(0)
    LatScale =     cPI * 6371000 / 180        ' converts latitude degrees to m
    LngScale = LatScale * CosD(oGeoPoint1.Latitude) ' converts longitude degrees to m
    Lat1 = oGeoPoint1.Latitude * LatScale
    Lng1 = oGeoPoint1.Longitude * LngScale
    For i = 1 To lstPoints.Size - 1
        Dim oGeoPoint2 As OSMDroid_GeoPoint = lstPoints.Get(i)
        Lat2 = oGeoPoint2.Latitude * LatScale
        Lng2 = oGeoPoint2.Longitude * LngScale
        dArea = dArea + (Lng2 - Lng1) * (Lat2 + Lat1) / 2
        Lat1 = Lat2
        Lng1 = Lng2
    Next
   
    Return Abs(dArea)
End Sub
 
Last edited:

RB Smissaert

Well-Known Member
Licensed User
Routine to calculate the area in squaremeters.

B4X:
Sub GetPolygonArea(lstPoints As List) As Double
    Dim i As Int
    Dim Lat1, Lat2, Lng1, Lng2 As Double
    Dim dArea, LatScale, LngScale As Double

    'Get the areas.
    Dim oGeoPoint1 As OSMDroid_GeoPoint = lstPoints.Get(0)
    LatScale =     cPI * 6317000 / 180        ' converts latitude degrees to m
    LngScale = LatScale * CosD(oGeoPoint1.Latitude) ' converts longitude degrees to m
    Lat1 = oGeoPoint1.Latitude * LatScale
    Lng1 = oGeoPoint1.Longitude * LngScale
    For i = 1 To lstPoints.Size - 1
        Dim oGeoPoint2 As OSMDroid_GeoPoint = lstPoints.Get(i)
        Lat2 = oGeoPoint2.Latitude * LatScale
        Lng2 = oGeoPoint2.Longitude * LngScale
        dArea = dArea + (Lng2 - Lng1) * (Lat2 + Lat1) / 2
        Lat1 = Lat2
        Lng1 = Lng2
    Next
   
    Return Abs(dArea)
End Sub
Thanks, will try that and report back.

RBS
 

RB Smissaert

Well-Known Member
Licensed User
Routine to calculate the area in squaremeters.

B4X:
Sub GetPolygonArea(lstPoints As List) As Double
    Dim i As Int
    Dim Lat1, Lat2, Lng1, Lng2 As Double
    Dim dArea, LatScale, LngScale As Double

    'Get the areas.
    Dim oGeoPoint1 As OSMDroid_GeoPoint = lstPoints.Get(0)
    LatScale =     cPI * 6317000 / 180        ' converts latitude degrees to m
    LngScale = LatScale * CosD(oGeoPoint1.Latitude) ' converts longitude degrees to m
    Lat1 = oGeoPoint1.Latitude * LatScale
    Lng1 = oGeoPoint1.Longitude * LngScale
    For i = 1 To lstPoints.Size - 1
        Dim oGeoPoint2 As OSMDroid_GeoPoint = lstPoints.Get(i)
        Lat2 = oGeoPoint2.Latitude * LatScale
        Lng2 = oGeoPoint2.Longitude * LngScale
        dArea = dArea + (Lng2 - Lng1) * (Lat2 + Lat1) / 2
        Lat1 = Lat2
        Lng1 = Lng2
    Next
   
    Return Abs(dArea)
End Sub
Shouldn't your 6317000 be 6378137, according to:
https://en.wikipedia.org/wiki/World_Geodetic_System ?

RBS
 

klaus

Expert
Licensed User
Sorry, it should be 6371000, I permuted the 1 and 7.
Amended in post#2.
I got the value from HERE, 6371 km is a kind of mean radius.
6378137 is the semi-major axis and 6356752 is the semi-minor axis.
 

RB Smissaert

Well-Known Member
Licensed User
Sorry, it should be 6371000, I permuted the 1 and 7.
Amended in post#2.
I got the value from HERE, 6371 km is a kind of mean radius.
6378137 is the semi-major axis and 6356752 is the semi-minor axis.
The area I am interested in is only small, latitude from about 52.45 to 52.65, so I will take the mean radius
of that and use that for all locations.

RBS
 

RB Smissaert

Well-Known Member
Licensed User
See the methods to calculate flat area and spherical area in the Navigation library https://www.b4x.com/android/forum/threads/navigation-library.16334/
Use flat area for area smaller the ~ 1 square km, spherical for larger area.
nav.SphericalArea2 doesn't seem to work with squares or rectangles, gives me zero:

B4X:
Sub GetPolygonAreaSpherical(lstPoints As List) As Double

 Dim i As Int
 Dim iPoints As Int
 Dim dArea As Double
 
 iPoints = lstPoints.Size - 1
 
 Dim arrPoints(iPoints, 2) As Double
 
 For i = 0 To iPoints - 1 'so don't include last point, which is same as first
  Dim oGeoPoint As OSMDroid_GeoPoint = lstPoints.Get(i)
  arrPoints(i, 0) = oGeoPoint.Latitude
  arrPoints(i, 1) = oGeoPoint.Longitude
 Next

 dArea = nav.SphericalArea2(arrPoints, iPoints, 0, 1)
 
 Return Round2(Abs(dArea), 4)

End Sub
Running the above Sub on a non-rectangle works fine.
Am I overlooking something?


RBS
 

derez

Expert
Licensed User
nav.SphericalArea2 doesn't seem to work with squares or rectangles, gives me zero:
...

Running the above Sub on a non-rectangle works fine.
Am I overlooking something?


RBS
Please check your data. I run this sub and get 150.0697679885843

B4X:
Sub test2
Dim points(4,2) As Double
    points(0,0) = 52.45
    points(0,1) = 30.10
    points(1,0) = 52.65
    points(1,1) = 30.10
    points(2,0) = 52.65
    points(2,1) = 30.20
    points(3,0) = 52.45
    points(3,1) = 30.20
    Log(nav.SphericalArea2(points,4,0,1))
End Sub
 

RB Smissaert

Well-Known Member
Licensed User
Please check your data. I run this sub and get 150.0697679885843

B4X:
Sub test2
Dim points(4,2) As Double
    points(0,0) = 52.45
    points(0,1) = 30.10
    points(1,0) = 52.65
    points(1,1) = 30.10
    points(2,0) = 52.65
    points(2,1) = 30.20
    points(3,0) = 52.45
    points(3,1) = 30.20
    Log(nav.SphericalArea2(points,4,0,1))
End Sub
Try this:

B4X:
Sub test2
 Dim points(4,2) As Double
 Dim dArea As Double
 points(0,0) = 52.597316
 points(0,1) = -2.150112
 points(1,0) = 52.597316
 points(1,1) = -2.147188
 points(2,0) = 52.594396
 points(2,1) = -2.147188
 points(3,0) = 52.594396
 points(3,1) = -2.150112
 dArea = nav.SphericalArea2(points,4,0,1)
 Log(dArea)
 Log(Round2(dArea, 4))
End Sub
Gives me NaN and 0


RBS
 

derez

Expert
Licensed User
Try this:

B4X:
Sub test2
 Dim points(4,2) As Double
 Dim dArea As Double
 points(0,0) = 52.597316
 points(0,1) = -2.150112
 points(1,0) = 52.597316
 points(1,1) = -2.147188
 points(2,0) = 52.594396
 points(2,1) = -2.147188
 points(3,0) = 52.594396
 points(3,1) = -2.150112
 dArea = nav.SphericalArea2(points,4,0,1)
 Log(dArea)
 Log(Round2(dArea, 4))
End Sub
Gives me NaN and 0


RBS
The area enclosed is less than 1 square km, so you should not use this method here. I decreased the distance slowly and got down to area of 0.2 , I guess the bearing calculation in the algorithm cannot handle it.
 

RB Smissaert

Well-Known Member
Licensed User
The area enclosed is less than 1 square km, so you should not use this method here. I decreased the distance slowly and got down to area of 0.2 , I guess the bearing calculation in the algorithm cannot handle it.
So, I should then use FlatArea2 first and see if the area is > 1 km2 and then use SpericalArea2?
Although it might a bit less accurate, in that case it makes sense perhaps to use the algo posted by Klaus.

There also seems to be a problem with SphericalArea2 if there are a lots of points:

B4X:
Sub test2
 Dim points(72,2) As Double
 Dim dArea As Double
 points(0,0) = 52.613914
 points(0,1) = -2.20366
 points(1,0) = 52.614383
 points(1,1) = -2.2019
 points(2,0) = 52.615191
 points(2,1) = -2.200913
 points(3,0) = 52.618057
 points(3,1) = -2.200012
 points(4,0) = 52.619073
 points(4,1) = -2.198381
 points(5,0) = 52.620037
 points(5,1) = -2.197265
 points(6,0) = 52.619933
 points(6,1) = -2.181859
 points(7,0) = 52.617719
 points(7,1) = -2.179069
 points(8,0) = 52.616911
 points(8,1) = -2.177224
 points(9,0) = 52.616442
 points(9,1) = -2.174949
 points(10,0) = 52.616259
 points(10,1) = -2.173275
 points(11,0) = 52.617588
 points(11,1) = -2.171688
 points(12,0) = 52.619047
 points(12,1) = -2.170572
 points(13,0) = 52.620506
 points(13,1) = -2.174091
 points(14,0) = 52.623633
 points(14,1) = -2.176408
 points(15,0) = 52.624362
 points(15,1) = -2.174434
 points(16,0) = 52.626967
 points(16,1) = -2.174649
 points(17,0) = 52.62741
 points(17,1) = -2.171344
 points(18,0) = 52.628686
 points(18,1) = -2.170958
 points(19,0) = 52.631161
 points(19,1) = -2.171044
 points(20,0) = 52.631812
 points(20,1) = -2.172203
 points(21,0) = 52.631578
 points(21,1) = -2.174992
 points(22,0) = 52.630796
 points(22,1) = -2.176194
 points(23,0) = 52.630796
 points(23,1) = -2.176194
 points(24,0) = 52.631526
 points(24,1) = -2.181429
 points(25,0) = 52.632698
 points(25,1) = -2.181816
 points(26,0) = 52.635511
 points(26,1) = -2.184047
 points(27,0) = 52.635849
 points(27,1) = -2.18585
 points(28,0) = 52.635068
 points(28,1) = -2.187695
 points(29,0) = 52.634339
 points(29,1) = -2.191171
 points(30,0) = 52.634339
 points(30,1) = -2.191171
 points(31,0) = 52.634573
 points(31,1) = -2.193188
 points(32,0) = 52.636448
 points(32,1) = -2.193918
 points(33,0) = 52.637829
 points(33,1) = -2.194862
 points(34,0) = 52.637959
 points(34,1) = -2.197008
 points(35,0) = 52.637438
 points(35,1) = -2.199625
 points(36,0) = 52.636136
 points(36,1) = -2.200269
 points(37,0) = 52.634339
 points(37,1) = -2.200312
 points(38,0) = 52.633193
 points(38,1) = -2.201428
 points(39,0) = 52.632724
 points(39,1) = -2.202887
 points(40,0) = 52.633193
 points(40,1) = -2.205934
 points(41,0) = 52.633375
 points(41,1) = -2.208251
 points(42,0) = 52.634964
 points(42,1) = -2.211728
 points(43,0) = 52.635328
 points(43,1) = -2.213487
 points(44,0) = 52.633974
 points(44,1) = -2.214903
 points(45,0) = 52.633036
 points(45,1) = -2.215032
 points(46,0) = 52.63288
 points(46,1) = -2.213873
 points(47,0) = 52.631838
 points(47,1) = -2.209753
 points(48,0) = 52.631421
 points(48,1) = -2.209067
 points(49,0) = 52.629911
 points(49,1) = -2.209024
 points(50,0) = 52.629129
 points(50,1) = -2.209367
 points(51,0) = 52.628556
 points(51,1) = -2.208681
 points(52,0) = 52.627306
 points(52,1) = -2.204303
 points(53,0) = 52.626394
 points(53,1) = -2.206707
 points(54,0) = 52.626915
 points(54,1) = -2.209239
 points(55,0) = 52.626759
 points(55,1) = -2.21044
 points(56,0) = 52.62556
 points(56,1) = -2.210569
 points(57,0) = 52.625665
 points(57,1) = -2.211899
 points(58,0) = 52.625665
 points(58,1) = -2.211899
 points(59,0) = 52.626576
 points(59,1) = -2.21323
 points(60,0) = 52.626238
 points(60,1) = -2.215075
 points(61,0) = 52.624883
 points(61,1) = -2.214946
 points(62,0) = 52.622617
 points(62,1) = -2.215161
 points(63,0) = 52.621887
 points(63,1) = -2.21486
 points(64,0) = 52.620532
 points(64,1) = -2.214517
 points(65,0) = 52.619464
 points(65,1) = -2.213916
 points(66,0) = 52.619829
 points(66,1) = -2.210226
 points(67,0) = 52.620975
 points(67,1) = -2.206792
 points(68,0) = 52.621575
 points(68,1) = -2.202973
 points(69,0) = 52.61949
 points(69,1) = -2.202286
 points(70,0) = 52.617041
 points(70,1) = -2.202973
 points(71,0) = 52.615113
 points(71,1) = -2.204217
 dArea = nav.SphericalArea2(points,72,0,1)
 Log(dArea)
 Log(Round2(dArea, 4))
End Sub

RBS
 

emexes

Well-Known Member
Licensed User
What areas are being measuring, and for what purpose? If the data above is typical, and the areas are less than a square degree, it's probable that flat areas (and straight-line distances) are accurate enough for your purpose.

If they're not accurate enough, then perhaps revisit the conversion of angles (longitude and latitude) to linear distances, because using an average radius is going to introduce errors up to the order of +/- 0.3% (for distance, and twice that for areas).

On the bright side, if you are just comparing areas or distances to see if one is smaller or closer than the other, then most of that error will cancel out anyway. Bonus!
 

RB Smissaert

Well-Known Member
Licensed User
What areas are being measuring, and for what purpose? If the data above is typical, and the areas are less than a square degree, it's probable that flat areas (and straight-line distances) are accurate enough for your purpose.

If they're not accurate enough, then perhaps revisit the conversion of angles (longitude and latitude) to linear distances, because using an average radius is going to introduce errors up to the order of +/- 0.3% (for distance, and twice that for areas).

On the bright side, if you are just comparing areas or distances to see if one is smaller or closer than the other, then most of that error will cancel out anyway. Bonus!
It doesn't have to be seriously accurate and I am not comparing area's.
My range of latitude is very small, so one radius for all locations is fine.

To me it looks there is a bug in nav.SpericalArea2 if there are a lot of points.

RBS
 

emexes

Well-Known Member
Licensed User
To me it looks there is a bug in nav.SpericalArea2 if there are a lot of points.
I think you're right. My first guess would be a float (single-precision) operation being used with small-difference-of-large-number maths.

My range of latitude is very small, so one radius for all locations is fine.
Bonus!

I stayed out of the radius-of-earth discussion, but one useful factoid I use all the time when close-enough is good-enough, is that the metre was originally defined as 10,000,000 metres = 90 degrees of earth circumference (from north pole to equator, through Paris). Thus for converting degrees latitude to distance you can use:

metres = 10000000 * latitude / 90

no Pi or hard-to-remember earth radius required :)
 

emexes

Well-Known Member
Licensed User
To me it looks there is a bug in nav.SpericalArea2 if there are a lot of points.
I did some tests, using a 1x1 degree patch near the equator, and:
1/ same problem occurs for just 4 points
2/ problem randomly comes and goes as patch is slowly enlarged from 1x1 to 1x2 degrees
3/ problem seems to be with the latitude
4/ when it does return results, the area should be increasing monotonically as the NS latitude size of the patch increases, but... check this out:
B4X:
latitude size         area

2.000000000152  NaN
2.000000000153  24749.007912058205
2.000000000154  24749.00791209434
2.000000000155  24749.007912166602
2.000000000156  24749.00791209434
2.000000000157  24749.007912166602
2.000000000158  24749.00791213047
2.000000000159  24749.00791213047
2.000000000160  24749.007912166602
2.000000000161  24749.007912166602
2.000000000162  24749.00791209434
2.000000000163  24749.00791213047
2.000000000164  24749.007912058205
2.000000000165  24749.00791213047
2.000000000166  24749.00791209434
2.000000000167  24749.00791209434
2.000000000168  24749.007912166602
2.000000000169  NaN
2.000000000170  24749.00791220273
2.000000000171  NaN
2.000000000172  24749.007912274992
2.000000000173  NaN
Hmm. Handy to know. Library author/creator Derez is still listed as a licenced user and active forum member; I'll let you have the honour of giving him the good news (if you haven't already).

:)
 

emexes

Well-Known Member
Licensed User
To me it looks there is a bug in nav.SpericalArea2
Had a bit of an epiphany over lunch, modified your code to randomly shift the latitudes up/down by ~1/10,000,000th of a degree (ie < 1 cm), figured that if each of those nudged 72 points has a 50% chance of landing in a "good place" then it should only take about 2^72 tests to find a set where all points are good and the spherical area is returned... at say 100,000 tests per second, that should only take... (grabs calculator) 1,496,427,638 years... hmm...

No worries, tried it anyway. That lead me to discovering that your polygon has three duplicate points, which I removed and then... voila! comes back with area that seems about right. I still think there's a problem in the library though, given that my simple 4-point patches did not contain duplicate points.
B4X:
** Activity (main) Create, isFirst = true **
4.409755228643426
4.4098
** Activity (main) Resume **
B4X:
Dim points(72, 2) As Double
Dim dArea As Double

points(0,0) = 52.613914
points(0,1) = -2.20366
points(1,0) = 52.614383
points(1,1) = -2.2019
points(2,0) = 52.615191
points(2,1) = -2.200913
points(3,0) = 52.618057
points(3,1) = -2.200012
points(4,0) = 52.619073
points(4,1) = -2.198381
points(5,0) = 52.620037
points(5,1) = -2.197265
points(6,0) = 52.619933
points(6,1) = -2.181859
points(7,0) = 52.617719
points(7,1) = -2.179069
points(8,0) = 52.616911
points(8,1) = -2.177224
points(9,0) = 52.616442
points(9,1) = -2.174949
points(10,0) = 52.616259
points(10,1) = -2.173275
points(11,0) = 52.617588
points(11,1) = -2.171688
points(12,0) = 52.619047
points(12,1) = -2.170572
points(13,0) = 52.620506
points(13,1) = -2.174091
points(14,0) = 52.623633
points(14,1) = -2.176408
points(15,0) = 52.624362
points(15,1) = -2.174434
points(16,0) = 52.626967
points(16,1) = -2.174649
points(17,0) = 52.62741
points(17,1) = -2.171344
points(18,0) = 52.628686
points(18,1) = -2.170958
points(19,0) = 52.631161
points(19,1) = -2.171044
points(20,0) = 52.631812
points(20,1) = -2.172203
points(21,0) = 52.631578
points(21,1) = -2.174992
points(22,0) = 52.630796
points(22,1) = -2.176194
'same as previous point
'points(23,0) = 52.630796
'points(23,1) = -2.176194
points(24-1,0)= 52.631526
points(24-1,1)= -2.181429
points(25-1,0)= 52.632698
points(25-1,1)= -2.181816
points(26-1,0)= 52.635511
points(26-1,1)= -2.184047
points(27-1,0)= 52.635849
points(27-1,1)= -2.18585
points(28-1,0)= 52.635068
points(28-1,1)= -2.187695
points(29-1,0)= 52.634339
points(29-1,1)= -2.191171
'same as previous point
'points(30,0) = 52.634339
'points(30,1) = -2.191171
points(31-2,0)= 52.634573
points(31-2,1)= -2.193188
points(32-2,0)= 52.636448
points(32-2,1)= -2.193918
points(33-2,0)= 52.637829
points(33-2,1)= -2.194862
points(34-2,0)= 52.637959
points(34-2,1)= -2.197008
points(35-2,0)= 52.637438
points(35-2,1)= -2.199625
points(36-2,0)= 52.636136
points(36-2,1)= -2.200269
points(37-2,0)= 52.634339
points(37-2,1)= -2.200312
points(38-2,0)= 52.633193
points(38-2,1)= -2.201428
points(39-2,0)= 52.632724
points(39-2,1)= -2.202887
points(40-2,0)= 52.633193
points(40-2,1)= -2.205934
points(41-2,0)= 52.633375
points(41-2,1)= -2.208251
points(42-2,0)= 52.634964
points(42-2,1)= -2.211728
points(43-2,0)= 52.635328
points(43-2,1)= -2.213487
points(44-2,0)= 52.633974
points(44-2,1)= -2.214903
points(45-2,0)= 52.633036
points(45-2,1)= -2.215032
points(46-2,0)= 52.63288
points(46-2,1)= -2.213873
points(47-2,0)= 52.631838
points(47-2,1)= -2.209753
points(48-2,0)= 52.631421
points(48-2,1)= -2.209067
points(49-2,0)= 52.629911
points(49-2,1)= -2.209024
points(50-2,0)= 52.629129
points(50-2,1)= -2.209367
points(51-2,0)= 52.628556
points(51-2,1)= -2.208681
points(52-2,0)= 52.627306
points(52-2,1)= -2.204303
points(53-2,0)= 52.626394
points(53-2,1)= -2.206707
points(54-2,0)= 52.626915
points(54-2,1)= -2.209239
points(55-2,0)= 52.626759
points(55-2,1) = -2.21044
points(56-2,0) = 52.62556
points(56-2,1) = -2.210569
points(57-2,0) = 52.625665
points(57-2,1) = -2.211899
'same as previous point
'points(58,0) = 52.625665
'points(58,1) = -2.211899
points(59-3,0) = 52.626576
points(59-3,1) = -2.21323
points(60-3,0) = 52.626238
points(60-3,1) = -2.215075
points(61-3,0) = 52.624883
points(61-3,1) = -2.214946
points(62-3,0) = 52.622617
points(62-3,1) = -2.215161
points(63-3,0) = 52.621887
points(63-3,1) = -2.21486
points(64-3,0) = 52.620532
points(64-3,1) = -2.214517
points(65-3,0) = 52.619464
points(65-3,1) = -2.213916
points(66-3,0) = 52.619829
points(66-3,1) = -2.210226
points(67-3,0) = 52.620975
points(67-3,1) = -2.206792
points(68-3,0) = 52.621575
points(68-3,1) = -2.202973
points(69-3,0) = 52.61949
points(69-3,1) = -2.202286
points(70-3,0) = 52.617041
points(70-3,1) = -2.202973
points(71-3,0) = 52.615113
points(71-3,1) = -2.204217
   
dArea = nav.SphericalArea2(points, 72-3, 0, 1)
Log(dArea)
Log(Round2(dArea, 4))
 

RB Smissaert

Well-Known Member
Licensed User
That lead me to discovering that your polygon has three duplicate points, which I removed and then... voila! comes back with area that seems about right.
Thanks for spotting those duplicates.
Should have checked.

RBS
 
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