How to define EXTENT

[Mark Cady]

Thanks Trevor:


Lets see if I understand it now!


  My trusty sectional is a paper map that non instrument rated pilots
use for navigation.
It did not come with a "world file" but it is at a 1:1,000,000 scale and
has navigation aids, roads, lakes, etc. on it.



Would this example cover one fourth of the world in decimal degrees?
(North of the equator and east of England)

0.16666667535901        - distance of one unit (pixel?)in the positive
direction (East)
0.0                                           - max X
0.0                                           - min Y
-0.16666667535901     - distance of one unit (pixel?) in the negative
direction ( West)
-179.91666666232049      -  Min X
89.91667605005205            - Max Y

If I want to create a map of aprox. the area of Minneapolis/St. Paul
the  EXTENT would be
(I'll show my work)

>From my paper map (Twin Cities Sectional - for you pilots)
                                Lat                Lon
NW corner is 45.5 N and 94W
NE corner is 45.5 N and 92.5W
SW corner is 44.5N and 94W
SE corner is 44.5N and 92.5W

EXTENT [xmin] [ymin][xmax][ymax]
EXTENT [94W][44.5N][92.5W][45.5N]
EXTENT[-94.000][44.500][-92.500][45.500]

for the .map file
EXTENT -94.000 44.500 -92.500 45.500

for the world file that goes with my raster layer ( mark.tif  - if it
was 600 X 600 pixels)

0.0025        - distance of one unit (pixel?)in the positive direction
(East) (94 - 92.5 ) / 600
-92.500     - Max X
44.500        - Min Y
-0.0025     - distance of one unit (pixel?) in the negative direction (
West) (94- 92.5) / 600
-94.00        -  Min X
45.500        - Max Y


Thanks for your help

Mark C.