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New Contributor
Posts: 4
Registered: ‎12-05-2019
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Why is given Scale not correct in Erdas Imagine 2018?

I noticed when I display an image in a viewer with the scale set to e.g. 1:20000, the image is actually displayed rather at 1:22000 (I calculated the true scale by dividing the number of visible pixels by the viewer width, which I measured with a ruler hold onto the display. On my colleague's display with a different screen resolution the scale discrepancy is even more extreme: the true scale seems to be 1:30000 instead of the given 1:20000! 

 

For further investigation, I measured the width of the viewer also with the Inquire Tool (Legacy) using the coordinate type 'Paper' (which I assume, should be the width on the display in inches): I found a discrepancy between my ruler measurement and the inquire tool measurement of about 3cm on my screen. On my colleague's screen it was even 17cm off!

When I calculated the viewer's scale with this digitally measured viewer width, it equaled 1:20000!

 

Further, I compared the image extent in Imagine with Esri ArcGIS, both at 1:20000. I could see, that the true scales differed.

 

My theory is, that Erdas Imagine calculates the scale using a wrong internal measurement of the viewer's physical width. 

Has someone noticed this scale issue, too?

 

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Posts: 364
Registered: ‎11-16-2015

Re: Why is given Scale not correct in Erdas Imagine 2018?

[ Edited ]

The scale is based on the "standardized rendering pixel size" which is 0.28mm. This standard size is used for instance by the OGC for some service standards.

The true pixel size of the screen is not used here to compute the scale, this is a kind of standardized scale. You can easily see that by viewing the image using the 1:1 tool (1 img pix = 1 pix screen) and changing the screen, the displayed scale will remain the same.

For example, displaying a 30m resolution image at 1:1 will always give you a scale of 1/107143 (=0.28/30000).