Calibrating Map Images
A calibrated image is a group of two files named equal except the extension:
– A map image (.jpg or .png)
– A text file (.map) that specify how the image pixels corresponds to real world coordinates
If you only have the map image in jpg or png format, you need to create it’s associated .map file. There are computer mapping softwares that can help you to create a .map file form a map image, as OZI Explorer.
In this post it is explained how to generate a .map file by two different ways:
– Within the tool itself
Calibrating Maps within The Fast Track
The NOAA’s New York Harbor nautical chart, will be used to illustrate this process.
First of all you need to choose a couple of reference points, first one should be close to the image upper right corner (NW), second one in the lower left (SE).
With an image editing software identify the pixels x,y coordinates of the selected reference points.
Create two waypoints with TFT with a long touch over the selected points in the map view.
Now you need to “map” the image pixels with the waypoints, select:
Menu -> Cartography -> Image maps -> Calibrate Image Map
select the first waypoint and introduce the reference point 1 x,y coordinates.
And then select the second waypoint and introduce the reference point 2 x,y coordinates
If everything is ok, then your image map should appear now in the map screen! Congratulations!
Manual .map generation.
A .map is a plain text file. For The Fast Track we only need to write 8 lines:
first 4 lines define 4 pixels in the image
MMPXY,<point number>, <pixel X coordinate>, <pixel Y doordinate>
next 4 lines define the coordinates of those pixels
MMPLL,<point number>, <pixel longitude>, <pixel latitude>
The number of the calibration points is defined in the sense of clockwise, starting from the northwest point, which is point 1, following by the northeast (2), the southeast (3) and finally the southwest (4)
In fact, The Fast Track only uses points 1 & 3 for its calibrations.
Once generated the .map file, copy both files .jpg & .map into the mobile device under the same path.
Test your map with:
Menu -> Cartography -> Map Image-> Open Map Image
The following file is an example of a calibration file for the New York harbor:
Enjoy your Sailing, Nautka Apps