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Lattitude and Longitude to X,Y

  1. Dec 8, 2009 #1
    I was writing a program to place a markers on a map of the United States, for a Civil War battlemap generator. I tried this:

    Assuming x,y and lat/long are superimposed yet different coordinate systems. For example, if a coordinate system with a unary equivalent to 12 on the other were to exist, express two concurrent points. Then use the distance formula:
    sqrt( (24+12)^2+(24+12)^2)/sqrt ( (2+1)^2+(2+1)) = 12 (as do all other concurrent values.)

    I found pixel where the four corners states meet (195,306), and a point in Florida. Using the formula above, with lat/long and x/y I calculated a conversion factor of about 14. However, when I did the four corners and the Lake Michigan. The conversion factors varied dramitcally. I thusly assume the the lines curve, even on a flat projection where Texas looks like its near Florida? My code went thus...
    Code (Text):

    const float refcoord[2]={36.998976, -109.045172};
    const float conversionfactor=0.17975951;
    const int refpix[2]={195,306};
    void drawID(float lat, float lon, FXint& x,FXint& y){
    If this is the problem I think it is, could someone link me to some trig help? Thanks alot.
  2. jcsd
  3. Dec 8, 2009 #2


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    The size of a degree of longtitude (x-axis) depends on your latitude (since the Earth gets smaller nearer the poles)
    The size of a degree of longitude is the same everywhere.

    So you need a different scaling factor in X and Y
  4. Dec 8, 2009 #3
    What is the sequence or function that would define this scaling factor for x? I figured out the ratio above on my own, but my math education at the moment, is limited (trig). Or how would I determine it?

    Thanks again.
    Last edited: Dec 8, 2009
  5. Dec 8, 2009 #4
    Try googling "longitude x latitude y".
  6. Dec 8, 2009 #5


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  7. Dec 9, 2009 #6
    Whenever I put in that calculation for (3.141592654/180)*Cos(0)*6367449 I get 111132.950017331. This must be wrong (by several factors of 10). Also, I get 0.0174.. whenever I do the more complex equation (with the polar and equatorial radii) with Cos(0). Cos(0) is, of course, 1.

    In fact I seem to be encountering .017 type stuff a LOT. Is that a constant... or something near a standard cos(x)?

    Thanks again...
  8. Dec 9, 2009 #7


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    There are about 111km/deg at the equator so thats probbaly the value for longtitude in metres.
    0.0174 sounds a lot like 1deg in radians - remember you computer/calcualtor is workign in radians
  9. Dec 10, 2009 #8
    Ahhhhhhhhhhhhhhhhhhhhhhhhh..... should've seen THAT coming.

  10. Dec 13, 2009 #9
    Because I am a glutton for pain, I have decided to carry this one through (if I can). Aside from the obvious (km means 1000) and the less than obvious (cos(x)*6....). I tried to develop a new conversion ration JUST going vertical (different Y on a fixed X)... but the inaccuracy is .08 every 10 degrees (ratio/ratio, basically.) Is a fixed conversion ratio for longitude even valid? Otherwise, would this piece of code do it?

    Code (Text):

    const float refcoord[2]={36.999, -109.05};
    const float conversionfactor=4.3; ///The AVERAGE conversion ratio
    const int refpix[2]={321,337};
    void drawID(float lat, float lon, FXint& x,FXint& y){
    x=refpix[0]-(refcoord[0]-lat)*(conversionfactor*((float(180/3.141)*cos(float(3.141/180)*lat)) ));
    It gets practice, it gets fairly close (within the size of my marker) for some locations (St. Louis). However, there is a magic threshold somewhere my X value becomes fairly massive (lake Michigan). (If anyone wants this threshold, I can figure it out by just systematically logging the output of the sub.)

    Oh... and thanks for the help.
  11. Dec 15, 2009 #10

    Most navigational charts are http://en.wikipedia.org/wiki/Lambert_Conformal_Projection" [Broken], which have two parallel lines of latitude secant to the globe and intersecting it. The result is a chart of least distortion for it's scale, and where straight lines drawn on the chart closely approximate great circle lines.

    If you're starting with an LCC, you can use http://en.wikipedia.org/wiki/Lambert_Conformal_Projection#Transformation"to convert spheric coordinates into the projection coordinates. This is useful, as this is what most electronic mapping software uses to pinpoint one's location on a digitized chart from GPS feeds.

    No worries! http://fer3.com/arc/img/106859.pre-comp%20pages%20from%20afpam11-216.pdf" [Broken](but it's the good kind). Be forewarned, however, as it's a 70 MB file.

    Check out page 395. I think you may find it helpful. :smile:
    Last edited by a moderator: May 4, 2017
  12. Dec 15, 2009 #11
    Last edited: Dec 15, 2009
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