Ezifield has the ability to display a single or multiple background images. Most common images are supported such as bitmap, jpeg's, tiffs etc. To display these images in the correct place they need to be georeferenced. This allows them to be displayed in the correct geographic place. A simple ascii world file is used. This simple ascii file is called a world file and is a de-facto standard bought out by ESRI.
The world file associated with the bitmap or jpeg is simply the raster file name with the letter w appended. For example a jpeg file called example.jpg would have an associated world file of example.jpgw
The world file for the jpg image of our office location is as follows
0.0000041372849
0.0000000
0.0000000
-0.0000034731573
151.73001070371
-32.9231253397952
The top line is the horizontal size of a pixel in longitudinal degrees. Line 2 and 3 are rotations about x and y axis. Windows mobile will not display rotated images so these need to be set at zero. Line 4 is the vertical pixel size in latitudinal degree's. Line 5 is the longitude of the centre of the pixel at the top far left of the file. Line 6 is the latitude of the center of the pixel at the top far left of the file.
If you have access to ESRI, Autocad Map or some other GIS systems then you already have the capability to produce the image and world file suitable as a background in Ezifield. If you don't then a handy free resource is Google Earth; or if you are a company that is using the background image as part of your day to day work then Google Earth Pro. The only downside with Google Earth images is that the associated world file is not produced.
Hand Calculating World File from Google Earth
If you view the image above this was captured in Google Earth. However Google Earth also gives you the smarts to hand calculate the world file. We need Google Earth to tell us the geographic position of the top left and bottom right of the position in lat,longs in decimal degrees. We also need to know the dimensions of the bitmap which is reported by windows explorer. You may need to right click the file and look at properties. This file is 1024 by 668 pixels.
From Google Earth we have the top left position at (151.730664,-32.923566) and bottom right at (151.734016,-32.923088).
Then horizontal size of a pixel is =(difference in longitude)/picture extents X = (151.734016-151.730236)/1024 = 0.00000369140625
Vertical pixel size = (-32.925316 + 32.923252)/668 = -0.000003089820
The accuracy of this system is directly proportional to how accurately you can obtain the coordinates of the top left and bottom right of the picture.
Using a Program to Calculate the World File
The free GIS program MapWindow GIS running the "ShapeToEarth" plugin will capture the jpg from Google Earth and also create the associated world file. However the accuracy is purposely degraded to fit the Google Earth "Terms of Use" We have found that the horizontal and vertical pixel size can be up to 10% out; so be aware. It appears to be accurate enough for use with a hand held GPS which is only good to +- 5 meters anyway.
(1024)/(