World Plus Image¶
To add a normal image, for example a jpg or a gif, you will need to provide a world file, a small text file that says where the corners of the image is.
This page specifically covers the combination of a normal image format with a “world” file. A few image formats have well known ways of encoding location information in the image “metadata” fields. As an example a TIFF file with spatial metadata is known as a GeoTIFF.
Image Files¶
An image file is recognized as being in the “World plus Image” format if it is part of a file set consisting of the following:
| Example | Description | |
| Image File | filename.jpeg | Contains the raster data |
| World File | filename.jgw | contains the “world file” information letting us know where to draw the raster on the map |
| PRJ File | filename.prj | Optional file that defines the coordinates used in the “world file” above |
World and Projection Files for an Image¶
The extension of a world file is the first and last letters of the image extension with w appended at the end.
| Format | Files |
| JEPG | jpeg, jgw with optional prj |
| TIFF | tiff, tfw with optional prj |
The contents of a world file are as follows:
| Line | Example | Definition |
| 1 | 0.0359281435 | Horizontal scale for a pixel |
| 2 | 0.0000000000 | Rotation for row, usually zero |
| 3 | 0.0000000000 | Rotation for column, usually zero |
| 4 | -0.0359281437 | Vertical scale for pixel, usually negative as rasters count down from the upper left corner |
| 5 | -179.9820349282 | Translation, usually “easting” location of the upper left pixel (ie column 0) |
| 6 | 89.9820359281 | Translation, usually “northing” location of the upper left pixel (ie row 0) |
The Translation information can be thought of as the location of the upper left pixel, this locaiton is provided in the units of the projections being used:
- For lat/long this location should be in the range -180,180 -90,90
- If your image is not in lat/long you will have to include a .prj file, which is a text file with the WKT definition of the coordinate system that should be used.
JPEG Example:¶
If your image is world.jpg then you need a world.jgw file and optionally a world.prj file.
An example world.jgw
0.0359281435
0.0000000000
0.0000000000
-0.0359281437
-179.9820349282
89.9820359281
An example world.prj is:
- ::
- GEOGCS[“GCS_WGS_1984”, DATUM[“WGS_1984”, SPHEROID[“WGS_1984”,6378137,298.257223563]], PRIMEM[“Greenwich”,0], UNIT[“Degree”,0.017453292519943295]]
Related reference