Re: high precision conversion troubles

Vladyslav Lazarenko <>
Fri, 10 Jul 2009 09:27:53 -0700 (PDT)
On Jul 10, 12:14 pm, cerr <> wrote:

Hi There,

I need to do some calculations with GPS coordinates and i get all
weird values and i can't see where i'm going wrong.
My code:
            // calculate the average xy coordinates out of th=


polygon's corner points
            double CinXAvg =((((sphericalpolygon *) curAppr=

oach->getCheckInPolygon())->getCornerPoints()->at(0).x +


ericalpolygon *) curApproach->getCheckInPolygon())->getCornerPoints()->at(1=
).x +


ericalpolygon *) curApproach->getCheckInPolygon())->getCornerPoints()->at(2=
).x +


ericalpolygon *) curApproach-


            double CinYAvg =((((sphericalpolygon *) curAppr=

oach->getCheckInPolygon())->getCornerPoints()->at(0).y +


ericalpolygon *) curApproach->getCheckInPolygon())->getCornerPoints()->at(1=
).y +


ericalpolygon *) curApproach->getCheckInPolygon())->getCornerPoints()->at(2=
).y +


ericalpolygon *) curApproach->getCheckInPolygon())->getCornerPoints()->at(3=

            //save the average value in a hash table so we ca=

n later

go ahead and find the outest polygons
            xy AvgCIN(static_cast<double>(CinXAvg/10000000),

    for (int i=0; i < AvgHashTable.size(); i++) {
        if (i==0) {
        else {
           if (AvgHashTable[i].x < TopLeft.x)
           if (AvgHashTable[i].y < TopLeft.y)
           if (AvgHashTable[i].x > BottomRight.x)
           if (AvgHashTable[i].y > BottomRight.y)
The explanation the first lines are GPS coordinates from a polygon and
i take the average from the 4 points to figure out the centre of the
polygon (I know, geometrically it's not 100% correct but I figured
this should be good enough for me/my polygons) anyways I multiply it
by 10,000,000 in order to get rid of the digits pst the decimal point.
Then after the division by 4 i get two decimal places back....
then i add the numbers to a map in order to be able to easily access
them. type xy is a class with two double variables, x and y. This is
how my globals are declared:
map<int, xy>AvgHashTable;
    xy TopLeft;
    xy BottomRight;
Then i wanna loop through all the elements and figure out the largest
and the smallest value...
somewhere my double values get screwed and i have all weird numbers in
BottomRight as well as in TopLeft

Now my source values are all in the format with 7 decimal numbers. And
in the end I have in TopLeft.x 49.213812599999997 and in BottomRight.x
I got 49.281173000000003... The y values look good for both... any
ideas why these numbers look so awkwardly?


Try fixed-point arithmetic (

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