Image processing, display online images, and google maps.

Hi AT (Altough his question is quite old, he recently mailed me)

I missed your question in March, but better late than never i guess

The code is quite messy/ written for a specific purpose and i do not have a generic map form, so i will just try to explain what you have to do.

First thing to know is that you can access google map tiles using an unique URL for each tile

An URL could be:

http://mt1.google.com/mt?n=404&x=329&y=792&zoom=6

The important things to see here is that you have an X coordinate, and an Y coordinate (or tile number) and a zoom level.

With
http://mt1.google.com/mt?n=404&x=0&y=0&zoom=17
You get a map of the entire world

By lowering the zoom, you get a more closeup-view of the earth. At zoom level 16 there are 4 tiles to cover the entire earth.

At zoom lever 15 there are 16 tiles

etcetera etcetera... (Each tile is split up into 4 tiles at the next level)

To instantly calculate the x and y coordinate for a position given it's GPS coordinates, you have to do some simple calculations. You should note that google uses a mercator projection, and looking that up on wikipedia should give you the required formula's.

A small but important snippet of code is the following:

ldecX := (180.0 + ldecLong) / 360.0;
ldecY := -ldecLat * Math.Pi / 180; // convert to radians
ldecY := 0.5 * Math.Ln((1+Math.Sin(ldecY)) / (1 - Math.Sin(ldecY)));
ldecY *= 1.0/(2 * Math.Pi);// scale factor from radians to normalized
ldecY += 0.5;

ldecY := Math.Floor(ldecY*POWER(2, 17-lintZoom));
ldecX := Math.Floor(ldecX*POWER(2, 17-lintZoom));

This basically gives you the x and y's given the GPS coordinates and a zoom level.

If you want to have a satellite image, you need a different URL. The idea is basically the same, but instead of X and Y coordinates, it uses a string of letters (qtrs).

You can use this to convert the X and Y numbers to this string of letters:

convertToSatAddress(x : Integer;y : Integer;zoom : Integer) : Text[30]
ltxtAddress := '';

FOR i := 1 TO (17-zoom) DO
BEGIN
   rx := x MOD 2;
   x  := x DIV 2;
   ry := y MOD 2;
   y  := y DIV 2;
   rxry := FORMAT(rx)+FORMAT(ry);
   CASE rxry OF
      '00' : ltxtAddress := 'q' + ltxtAddress;
      '01' : ltxtAddress := 't' + ltxtAddress;
      '10' : ltxtAddress := 'r' + ltxtAddress;
      '11' : ltxtAddress := 's' + ltxtAddress;
    END;
END;
ltxtAddress := 't' + ltxtAddress;

EXIT(ltxtAddress);

The resulting string should be appended to:
http://kh.google.com/kh?v=3&t=

Creating somthing like:
http://kh.google.com/kh?v=3&t=tqst

Where the zoom level is defined by the number of characters in the string.

---

After getting the URL, you need to download it to your PC, and since 1 tile is a bit small, i decided to stitch 9 tiles together.
And after that it has to be coverted to the BMP file format.

The solution for that is to use the opensource Imagick library.
Is consists of a couple of very useful commandline utilities, that can, amongs many other things, do exactly what we want.

You can put the commands in a Batch script file, or use multiple SHELL calls.

I won't explain this in too much detail because:
1. Imagick is quite easy to learn, and there are many examples on the website.
2. I use an outdated version with slightly different syntax.

****Important note
Use the following parameters

"-units" "PixelsPerInch" "-density" "96x96"

because this will make Navision use the correct size
(Navision uses actual size (such as mm or inch) instead of pixel sizes, so this will make sure it is displayed properly)

---

But there is still room for improvement:
1. Crop the image (use more tiles, and cut a bit of at the edges)
This will center the map at the given GPS position. (Otherwise the center is somewhere in the middle image)
2. Use a cache. Google bans users which request to many images

****Important note
this is al reverse engineerd, so i don't know what googles view on this is,
but i guess that if you want to make very heavy or commercial use of it, you should just buy Mappoint. In all cases: i am not responsible, and you should consult the google maps policy.

3. put icons on top of you map.
Maybe this will help:

SatAdressToPixel(ldecLat : Decimal;ldecLong : Decimal)
ldecX := (180.0 + gdecLong) / 360.0;
ldecY := -gdecLat * Math.Pi / 180; // convert to radians
ldecY := 0.5 * Math.Ln((1+Math.Sin(ldecY)) / (1 - Math.Sin(ldecY)));
ldecY *= 1.0/(2 * Math.Pi);// scale factor from radians to normalized
ldecY += 0.5;

ldecX2 := (180.0 + ldecLong) / 360.0;
ldecY2 := -ldecLat * Math.Pi / 180; // convert to radians
ldecY2 := 0.5 * Math.Ln((1+Math.Sin(ldecY2)) / (1 - Math.Sin(ldecY2)));
ldecY2 *= 1.0/(2 * Math.Pi);// scale factor from radians to normalized
ldecY2 += 0.5;

lintX := (Math.Ceil((ldecX2*POWER(2, 17-gintZoom) - (ldecX*POWER(2, 17-gintZoom)))*256)+256);
lintY := (512-(Math.Ceil((ldecY*POWER(2, 17-gintZoom) - (ldecY2*POWER(2, 17-gintZoom)))*256)+256));

MESSAGE('X:%1, Y:%2', lintX, lintY);

4. Use 'Windows Script Host Object Model'.WshShell to hide the popup

---

Finally it is important to know that it won't plot routes.
And a final note is that the string contains a version number which might be subject to change, just like rest of the URL. Thus altough it has not changed for quite some time, you should never use it for a critical buissness process.

AT wrote:

Thank you for your detailed post. One more question:
How did you get the images to your PC? Is it necessary to open your browser?

I use the 'convert' command from the ImageMagick library. This command can take URL's as parameters, so no browser is required.

Lichtens wrote:

tI ried to convert your code in C++, but the tile i get is just the wrong one. Can anyone help me what is wrong with the calculation?

Not the calculation is wrong, but the coordinates are. If you want to get a tile near stuttgard, your Longitude should be positive (+9.6 instead of -9.6)

To get coordinates for certain locations, or vice versa, i used the following website. It is in dutch, but it might still be useful:
http://www.gpscoordinaten.nl/bepaal-gps-coordinaten.php