elec maps .. how do you find out what scale they are
Submitted: Monday, Sep 15, 2008 at 07:27
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Member - Drew T (Melbourne)
i have a number of electronic maps (that i use with oziexplorer). is there any way that i can
check whether a map is say 25k or 50k as i have some that i think are labelled incorrectly. in some cases the map file name tells me, but not in all cases. is this info within the *.map file & if so hao do you read them?
Reply By: The Explorer - Monday, Sep 15, 2008 at 13:25
Monday, Sep 15, 2008 at 13:25
Hello - can be difficult to back calculate scale of a scanned map if the information isn't displayed on the map itself. It certainly isnt automatically recorded by OziExplorer. OziExplorer does have info on metres per pixel (use 4th icon from right at top of screen) ...but this will be dependent not only on the scale of the original map but the resolution at which the map was scanned.
Some mapping standards do exist though -
well I think they do.
The 25K maps I have cover an area of ~12km x ~12km. Metres per pixel is ~3 when scanned at 200dpi.
The 50K maps cover about ~26 km x ~26km. metres per pixel is about 6 at 200dpi ......and 100k are double this again (~52km x ~52km and 12 or 13m/pixel at 200dpi).
It is possible to create any scale map over any area at any resolution though so these figures just relate to 200dpi scans of the average paper map you buy from the map
shop.
Cheers
Greg
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Reply By: Member - Alan H (QLD) - Monday, Sep 15, 2008 at 13:37
Monday, Sep 15, 2008 at 13:37
It is possible to measure distance on OziExplorer so if you have a known distance on the map you should be able to work scale.
Alan
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Follow Up By: Member - John and Val W (ACT) - Monday, Sep 15, 2008 at 15:44
Monday, Sep 15, 2008 at 15:44
Not too sure what you really mean as scale on a paper map will vary in an electronic map format depending on a few variables including the size of your screen. Use the range rings in the moving map function, or the line feature to measure distance for any given map on your particular screen.
HTH,
Val
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Follow Up By: The Explorer - Monday, Sep 15, 2008 at 17:17
Monday, Sep 15, 2008 at 17:17
Drews initial post is slightly ambiguous as its not really clear if he wants to know the scale of the original map (see info I provided) or the scale of the map when displayed on PC. I took him as wanting to know the scale of the original map before scanning.
When on the PC the map has no set scale as it will change as you zoom in and out e.g. 1cm across the screen may equal say 30metres at 100% zoom...but at 200% zoom 1cm across the screen will equal only 15 metres i.e. completely different scale.
Cheers
Greg
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Follow Up By: Member - Alan H (QLD) - Monday, Sep 15, 2008 at 17:25
Monday, Sep 15, 2008 at 17:25
What I mean is if you measure a distance of 1 kilometre on the map and physically it is 1cm on the screen then the map is showing 1:100000.
1km being 4cm on the map is 1:25000 etc.
I have measured the
grid on a 25000 map.
It is 40mm on a physical map
it is 100mm on the same map on Oziexplorer at 100%
Ozi measures this distance as 1km so it must know the scale etc to work it out
It depends on whether you talk about known scale maps or self scanned ones. They may or maynot have a
grid.
Alan
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Follow Up By: Mike Harding - Monday, Sep 15, 2008 at 18:30
Monday, Sep 15, 2008 at 18:30
Alan H:
That's what I would have though... but it doesn't seem to be that way according to my tests.
Clearly, OziEx understands the map's scale otherwise it's couldn't calculate distances and, iirc, when one imports a map into OziEx one has to supply lat/long co-ordinates for a number of points on the map so OziEx must use a mathematical model based upon lat/long to do it's calculations.
It _must_ be possible to solve this question (I think! :) but I'm damned if I can figure out how to do so at the moment and that is annoying me :)
Mike Harding
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Follow Up By: The Explorer - Monday, Sep 15, 2008 at 18:38
Monday, Sep 15, 2008 at 18:38
"It _must_ be possible to solve this question "
Now Im getting confused - What is the question you are trying to solve?
Cheers
Greg
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Follow Up By: Mike Harding - Monday, Sep 15, 2008 at 19:40
Monday, Sep 15, 2008 at 19:40
It's obvious! "Life, the universe and everything" :)
or
Reverse calculating the scale of a soft map?
Mike Harding
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