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point in polygon function and kml placemark test

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Bascom Member

Joined: 10 Feb 2014
Posts: 66
Location: Melbourne

PostPosted: Sat Nov 30, 2019 12:00 pm    Post subject: point in polygon function and kml placemark test Reply with quote

This is a work in progress, it works but the poly point array must start and end with the same coords.
I'm having a hard time getting this to happen in the next loop. ie closing the polygon

taken from one of the many pnp ray tracing algorithms on line
as it is now, its enough for what i need.

The test is just a static one - you'll need to capture the serial output and send it to a kml file
YAT (yet another terminal) works well for this

Function PIP(byval X as single , byval Y as single) as byte
  ' index is the polygon point array count
  ' fencelon() and fencelatt() global arrays of the x,y coords    -  fencelon is the "Y" axis
  ' x,y are the test point coords - x=latt y=lon

   local f1 as single, f2 as single,f3 as single,f4 as single,f5 as single  ,f6 as single
   local cn as byte
   local i as byte, j as byte    ,q as byte,r as byte

   i=1  :   cn = 0
 index = index-1
   for i = 1 to index
      q=0  :    r=0
      if  fencelon(i)< Y And fencelon(J)>= Y then q = 1
      if  fencelon(J)< Y And fencelon(i)>= Y then r=1
      If q=1 or r=1 then
         f1 = Y - fencelon(i) :  f2 = fencelon(j) - fencelon(i)  :  f3 = fencelatt(j) -  fencelatt(i)
         f4 = f1/f2
         f5 = f4 * f3
         f6 = fencelatt(i) + f5
         if f6 < X Then  cn = cn+1
      End If
   next i
 index = index+1

   cn = cn mod 2
   if cn > 0 then cn =1  ' even numbers are outside the polygon

   PIP = cn
End Function


sub PIPtestKML(byval  OffSet as  single, byval resolution as single)
'decimal place / grid resolution / offset
'2        0.01        1.1132 km
'3        0.001        111.32 m
'4        0.0001        11.132 m
'5        0.00001        1.1132 m
'go easy with these - easy to create many millions of points, google earth will have a hissy fit.
' check the terminal program for max number of displayed lines.
'have been using YAT terminal for this test, - need more than 80 columns and can change the buffered displayed lines easy.

'find  max lat and min latt, max lon min lon from stored fencelatt,lon array and add (offset) to them
'this test wont work accross the equator or the prime meridian

   local n as byte    ,m as byte
   local maxlon as single, minlon as single, maxlatt as single, minlatt as single  , original as single

 'set the focus point for initial Earth view and kml header stuff
   print  "<?xml version=";chr(34);"1.0";chr(34);" encoding=";chr(34);"UTF-8";chr(34);"?>"
   print  "<kml xmlns=";chr(34);"";chr(34);
   print " xmlns:gx=";chr(34);"";chr(34);">"
   print  "<Document>"
   print  "<LookAt> <longitude>";
   print fencelon(1);
   print  "</longitude> <latitude>";
   print fencelatt(1);
   print  "</latitude> <range>1300.000000</range> </LookAt>"
   print  "<Folder>"

  ''' gegerate the FENCEPOINTS    /  POLYPOINTS
   for n = 1 to index
      print  "<Placemark>"
      print "<name>";"FP";n;"</name>"
      print"<Point> <coordinates>"
      print fencelon(n);
      print ",";
      print fencelatt(n);
      print  "</coordinates></Point></Placemark>  "
   next n

 'find max and mins then add the offset value to generate the test box
   maxlatt = fencelatt(1) :   minlatt = fencelatt(1):   maxlon = fencelon(1) :  minlon = fencelon(1)
   for n = 1 to index
      if maxlatt <= fencelatt(n) then maxlatt = fencelatt(n)
      if minlatt  >= fencelatt(n) then minlatt  = fencelatt(n)
      if maxlon <= fencelon(n)  then maxlon = fencelon(n)
      if minlon  >=  fencelon(n) then minlon = fencelon(n)
   next n
   minlatt = minlatt - offset
   maxlatt = maxlatt + offset
   minlon = minlon - offset
   maxlon = maxlon + offset

  ''' run the test grid through the point in polygon(pip) function and generate placemark pins
   '' original = minlatt
   while minlatt < maxlatt and minlon < maxlon    'this sets the test boundry box - it's just easier to do a test square
      original = minlatt
      while minlatt < maxlatt   '   print the x axis
         m =  pip(minlatt, minlon)
         if m< 1 then    ' just display the outside points
            print  "<Placemark>"
            print "<name>";m;"</name>"
            print" <Point><coordinates>"
            print minlon;
            print ",";
            print minlatt;
            print  "</coordinates></Point></Placemark>"
         end if
         minlatt = minlatt + resolution
      minlatt = original
      minlon = minlon + resolution
   print  "</Folder></Document></kml>"

   loop   '  stop here -  need time to save to file in terminal
end sub

sub   preload()
   fencelatt(5)=-37.702547    'start repeat

   fencelon(5)=147.840474   'start repeat

   index = 5

end sub


call PIPtestKML(0.0005,0.00005) generated the kml below
with the preload sub lat lon values - for an idea of resolution

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Joined: 09 Apr 2004
Posts: 5070
Location: Holland

PostPosted: Thu Dec 05, 2019 9:59 am    Post subject: Reply with quote

thank you for sharing. it looks great, i needed some time to understand what it is Smile

i use kml files too when i walk to make a log file which i later look at with google earth.
I also did some tracking apps with gsm/wifi.
But it took me some time to understand the purpose. this is not just a square but a polygon.
great for fencing.


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Bascom Member

Joined: 30 Nov 2005
Posts: 158

PostPosted: Wed Aug 19, 2020 6:21 pm    Post subject: Reply with quote

Than you for publish this great example! : )

I was in the need of doing the same polygon test for one of my projects.

However, reading in detail the author page, I noticed the I & J must evaluate all the vertex.
I compared the original Author code in C++, also the original code of the same Author in Fortran.

in the function
Function PIP(byval X as single , byval Y as single) as byte

You have

index = index-1
J = index

when the function starts, so this leave last vertex out of the loop.

Maybe this is the reason you have issues about need to close the polygon (repeat the first vertex);
in fact the function do not need to repeat the first vertex, once it needs to cycle thru all the vertex.
The vertex order CW or CCW does not matter.

i.e. having a rectangle, you need to pass only 4 vertex to the function (index = 4) and not repeat the first one (index = 5).

So my 0.5 cents are a couple of things need to change:

1. do the loop from 1 to index (do not subtract 1 from index )
2. assign the value of J = N at the beginning, before the loop starts.

This way, the first loop iteration will evaluate I=1, J=N so effectively evaluating first and last vertex.
The rest of the loop iterations evaluates what else is in between.

It is working great.

Example: testing rectangle with 4 vertex:

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