Meters per Pixel in World Files

walterboyd

This is a puzzle.

I created a georeferenced Jpeg in Global Mapper. The world file (.jgw extension) clearly indicates 1 meter per pixel vertically and horizontally. When I measure the image in Global Mapper using the measure tool, it's 2678 meters wide and 2665 meters tall. But the JPG pixel dimensions are 3197 x 3182.

Why the discrepancy??? Shouldn't the pixel size and meters measured be the same?

Sample files are attached.

Cheers,
Walter

walterboyd

Well nevermind that question...seems the Measure Tool simply isn't reporting good data. Look at this screen shot - there is a big difference between the key and the Measure Tool distance. Was this a known bug in v11? I couldn't find anything about it.

MeasureToolBug.jpg

global_mapper

What projection are you viewing in? Some are heavily distorted in one direction vs. the other, so Y may be scaled one way, then X the other. For example X is heavily distorted in Mercator and Geographic, among others. There is no such thing as a perfect scale bar on a 2D map since the earth isn't flat, but if you change your view projection to something like UTM you can get much less distortion so long as your map covers a relatively small area.

Thanks,

Mike
Global Mapper Guru
gmsupport@bluemarblegeo.com
http://www.bluemarblegeo.com/products/global-mapper.php

walterboyd

Mike, thanks for the reply. This is Mercator. I understand how a projection can distort an image. And for this reason I would assume the Distance Scale would be approximate and the Measure Tool should be a reliable tool for measuring distance on a map. But, the opposite seems true in this case. The distance scale seems correct; when I compare my original image to the built in Distance Scale, the pixels do reflect 1m x 1m. But the Measure Tool is very incorrect.

Any other ideas?

Thanks,
Walter

global_mapper

Walter,

It should be the opposite where the Measure Tool is extremely accurate for true ground distance and the Distance Scale is what cannot be correct. Keep in mind that you cannot use the X and Y Mercator meters coordinates to get an accurate distance. So a pixel that is 1m x 1m in Mercator coordinates is likely something potentially much different than 1m x 1m in actual ground truth.

Thanks,

Mike
Global Mapper Guru
gmsupport@bluemarblegeo.com
http://www.bluemarblegeo.com/