# [OpenLayers-Users] Point in parrallel with line formula

newbie shai_mus at hotmail.com
Thu Apr 30 10:46:12 EDT 2009

```Dear Chris,
So once I have I have decide whether is +1 or -1 then I must
apply the other formula. So must I still apply the first formula to decide
+1 or -1 or can I just go to the second formula. Secondly I still cant
really see the implementation of the formula. So is there any example
implementation of this formula cause I dont know to how to get the answer in
metres? Thank you.

>
> +1 means to the left
> -1 means to the right
>
> before calling the 'ccw' function, you'll want to order p0 and p1 so
> that p0 has the smaller latitude
>
> To get the distance on the left or right, use the formula linked from my
> second response. It finds the closest point, on the line to the other
> point, and then you need to take the distance between the two. If you
> use Pythagorean theorem, it will give you distance in whatever
> measurement  the projection of your map is in.
>
> newbie wrote:
>> Dear Chris, I am a bit confuse. So the d will be in what measurement
>> is it meter or km ? Then if is + or - what is the difference too.
>> Thank you.
>>
>>     Here's a C++ function (Should be easy to convert to JavaScript) /*
>>     Taken from Robert Sedgewick, Algorithms in C++ */ /* returns
>>     whether, in traveling from the first to the second to the third
>>     point, we turn counterclockwise (+1) or not (-1) */ int ccw( Point
>>     p0, Point p1, Point p2 ) { int dx1, dx2, dy1, dy2; dx1 = p1.x -
>>     p0.x; dy1 = p1.y - p0.y; dx2 = p2.x - p0.x; dy2 = p2.y - p0.y; if
>>     (dx1*dy2 > dy1*dx2) return +1; if (dx1*dy2 < dy1*dx2) return -1;
>>     if ((dx1*dx2 < 0) || (dy1*dy2 < 0)) return -1; if ((dx1*dx1 +
>>     dy1*dy1) < (dx2*dx2 + dy2*dy2)) return +1; return 0; } >From this
>>     post on GameDev.net
>>     http://www.gamedev.net/community/forums/topic.asp?topic_id=457450
>>     Note it will also return 0 if the point is precisely on the line.
>>     The deals with turn directions, so it depends on the ordering of
>>     the points on the line. To get around this, you'll want to swap p0
>>     and p1, if if p1 is below p0. (If p1's latitude is smaller than
>>     p0's) newbie wrote: > Dear All, > Just say I got a pair of lat and
>>     long p1 and p2 and I draw a line > based on both the points. So
>>     now I want to know if a particular point in the > parallel range
>>     of say 50m either on left or right sides of the line. How can > I
>>     decide this what is the formula please ? >
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>>
>>
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