Difference between revisions of "Background Image"

Revision as of 17:08, 11 August 2009

What it is

A background image data item consists of an image and its geo-reference, given in the *.img file:

IMAGE_FILE                    : C:\temp\389_.jpg  --> Path to the image file
X                             : 0                 --> Coordinate X of the Lower Left Corner
Y                             : 0                 --> Coordinate Y of the Lower Left Corner
WIDTH                         : 16008             --> Width in distance units
HEIGHT                        : 10011             --> Height in distance units
MIP_MAPPED                    : 1
MAINTAIN_RATIO                : 1
ALPHA_VALUE                   : 1

Import ESRI geo-referenced image in MOHID GIS

ESRI geo-referenced images are comprised of two files: the image (e.g. image.jpg) and the "world file" with the coordinates (e.g. image.jpgw):

3                       --> pixels/unit distance X
0.000000000000
0.000000000000 
-3                      --> pixels/unit distance Y
-111989.9               --> Coordinate X of Upper Left Corner
-70266.15               --> Coordinate Y of Upper Left Corner

This are the actions to import geo-referenced images to MOHID GIS

1. Add New Item.
2. Background Image; choose file name.
3. In this new layer created, if a window does not appear automatically, go to Properties (F4, or right-click, or Menu Data Items/Properties).
4. Choose the image file (browse)
5. Click "Import ArcView GeoRef" to import automatically the "world file"
6. Data Items -> Save All, in order to save the just created .img file

Now, everytime you want to open that background image go to Open->Background Image (type of file) and select the .img file created.

Note: If loading the file fails and all that you see is a white image (but in the right place) the problem is the picture resolution is too high for Mohid Gis to load it. Lower the resolution (as an example a resolution of 4085x2554 was the limit for loading). If you have this problem do not resize to this value but do a shorting ratio (proportional to width and height) that produces resolution close (or lower) to his one.

Alternatively to this process you can convert by hand the world file to Mohid GIS img file,

• WIDTH (HEIGHT) = number of width (height) pixels time pixels/unit distance X (Y). The pixel information of the image can be seen in GIMP, for example.
• The Y coordinate must be decreased by height pixels times pixels/unit distance Y to go from Upper Left Corner (ESRI) to Lower Left Corner (Mohid GIS)
• Write the path to the file in IMAGE_FILE
• open the .img file created in MOHID GIS as a background image (file type).