GPSFS, Java and some troubles

[matrix681]

Hello,

im writing a little programm with java, where i want to see my position on a map.
so i decide to use GMDL.

first i have to thank for this great tool :thumbup:
i also have to thank for the detailed description of GPSFS at the end of the sourcefile :D
(but there seems to be a little error. there says, that the IDX begins with the highest zoom level, but it begins with the lowest zoomlevel :confused: )



however, i now can draw the map at my current position. but i have trouble with the exact positioning on the drawed map.

for better understanding a little example:
My GPS-Position is: 48.3767 16.2426

if i search this coordinates on maps.google.com i get the exact position on the freeway:
http://maps.google.com/maps?f=q&hl=en&geocode=&q=48.3767+16.2426&ie=UTF8&ll=48.376706,16.242599&spn=0.009193,0.016673&z=16&iwloc=addr&om=1

but if i draw the map on my own, im a little bit outside or on the border of the freeway:
http://stud3.tuwien.ac.at/~e0025170/fotos/gpsfs_position.jpg


i don't know much thing about the projection. i just 'translate' the code to get the gps-position of the downloaded map from the X-Y pair from the sourcecode of mapthis.
(getLatLong, getTileCoord, initLatLong)



Any idea, why the position could be not exact?

I do following to compute the position on the map:

1) i compute the coordinates of the upper-left and the lower-right corner of the image:

xLeft = getLatLong( (int)X, (int)(Y-1), 0)[1];
xRight = getLatLong( (int)(X+filesPerLine[zoomlevel]), (int)(Y+filesPerLine[zoomlevel]-1), 0)[1];
yTop = getLatLong( (int)X, (int)(Y-1), 0)[0];
yBottom = getLatLong( (int)(X+filesPerLine[zoomlevel]), (int)(Y+filesPerLine[zoomlevel]-1), 0)[0];

2) i find the 256*256 part, where my incoming GPS-Position is and draw it, with the surrounding parts in a 768*768 image.

3) i compute the position in this little image, and get a 300*300 pixel part of it.

4) my position should now be exact at the center of this 300*300 pixel, but it's not :(





sorry for my bad english and thanks in advance,
Ahmet Y.

[Codaz]

Your stud.tuwien.at image isn't working.

[deniska]

It's hard to speculate on what's going on without seeing the actual code.. Perhaps that little error comes from assumtion of linear dependency between lon/lat and the coordinates inside that tile.
Or perhaps, some error kicks in when the JRE rounds things up..

[deniska]

MapThis! seems to render the coordinates just like google maps (see below).. so you error comes from either different round up algorithm or some faulty logic..

[matrix681]

It's hard to speculate on what's going on without seeing the actual code.. Perhaps that little error comes from assumtion of linear dependency between lon/lat and the coordinates inside that tile.
well, i really assume that lon/lat are linear in a tile. :rolleyes:

do you have any helpfull links for me, where i can "easily" learn how to compute the exact position on the mercator projection? :)
(or a good introduction into this area)



do i have to use these two methods from your main.c ?


//puts x value in Coord.lon and y value in Coord.lat
Coord getTileCoord(double lat, double lon, int zoomLl ) {
int x,y;
int zoomLevel=zoomLl;
if (zoomLevel<0) zoomLevel=zoomLevel*(-1)+17;
x= (int)(bitmapOrigo[zoomLevel] + lon * pixelsPerLonDegree[zoomLevel]);
x/=256;
double e = sin(lat * Wa);

if (e > 0.9999) {
e = 0.9999;
}

if (e < -0.9999) {
e = -0.9999;
}

y = (int)(bitmapOrigo[zoomLevel] + 0.5
* log((1 + e) / (1 - e)) * -1
* (pixelsPerLonRadian[zoomLevel]));
y/=256;

Coord cc;
cc.lon=x;
cc.lat=y;
return cc;

}

//puts x value in Coord.lon and y value in Coord.lat
Coord getXY(double lat, double lon, int zoom) {
Coord c;
long x,y;
int zoomLevel=zoom;
if (zoomLevel<0) zoomLevel=zoomLevel*(-1)+17;

x= (long)(bitmapOrigo[zoomLevel] + lon * pixelsPerLonDegree[zoomLevel]);
x/=256;
double e = sin(lat * Wa);

if (e > 0.9999) {
e = 0.9999;
}

if (e < -0.9999) {
e = -0.9999;
}

y = (long)(bitmapOrigo[zoomLevel] + 0.5
* log((1 + e) / (1 - e)) * -1
* (pixelsPerLonRadian[zoomLevel]));
y/=256;
c=getLatLong(x,y,basezoom);

c.lon=(x-ftx)*TILE_SIZE+(lon-c.lon)*TILE_SIZE/c.lonWidth;
c.lat=(y-fty)*TILE_SIZE+(c.lat-lat)*TILE_SIZE/c.latHeight+TILE_SIZE;
return c;

}

[deniska]

check out this page: http://mapki.com/wiki/Knowledge_Base

They should have some java api samples already