Plotting geo-referenced photos to scale.

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  • Updated 7 years ago
I have a question about geo-referenced images, resolution, and print scale.

Geo-referenced Aerial photographs are scaled by the actual ground coverage of each pixel. When enlarging these images for plotting, I need tight control of the resolution. At times when I crop these images, the resolution/scale changes without my knowing about it. I "re-sample" images to try controlling the scale, but it's not working. Any suggestions?
Photo of Dan Spurling

Dan Spurling

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Posted 7 years ago

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Photo of Dan Spurling

Dan Spurling

  • 3 Posts
  • 0 Reply Likes
this is the current workflow for plotting images in our shop.
AMGL = Above Mean Ground Level, this is ground resolution, how much ground is covered by one pixel in the image.
DMC = Digital Mapping Camera

Scaling / Cropping In Photoshop from Full Frame DMC 8-bit images.

Using the Spreadsheet that corresponds to the AMGL of the flown mission, find
the requested plotting scale.

Find the “resulting resolution” that corresponds to that plotting scale.

Open the full frame DMC image you want to use

Go to Image / Image Size, the image size should read 2116.692 dpi with a width of 3.628” and a height of 6.531”. This is your original, full resolution / full frame size for a DMC image. Make sure that the resolution box reads out dots per INCH. Not per centimeter. This is crucial. When opening a DMC image in photoshop, the file resolution readout defaults to dots per centimeter and you must change it to inches.

In the Image / Image Size dialog box, make sure that all three boxes at the bottom are unchecked, so that width, height and resolution are all linked. In the resolution box, type in the “resulting resolution” that corresponds with the desired scale on the spreadsheet. This will cause your DMC image to now be at the desired scale. The width and height will change accordingly. They should now reflect the “Along Track and Cross Track Enlargement size” from the spreadsheet. The next step is to crop down to just what you want to plot.

In photoshop, go to File, new and create a new file. Make the new file to same resolution as the resized DMC image / “resulting resolution” from the spreadsheet. The width and height of this file should be whatever you want your final plot to be.

Once the file is made, go to “select, select all”. This will give you a selection box which runs the perimeter of the new file.

Select the “marque tool” (the tool that looks like a rectangular box of dashed lines).

Drag the selection from the file you just created, into the DMC image

The selection will appear as a moveable object, and it will be set to the size that you want your final plot to be. Drag this to the area of the image you wish to crop down to.

On the main menu, go to “Image / Crop”. This will crop your image down to the area you selected.

Some things to remember about this method:

Remember, any inconsistencies with resolution, or width and height will result in an incorrect scale. So double check that the final resolutions of both the DMC image and the file you create are the same. Once you have created your selection and brought it into the DMC image, adjusting the boxes width or height will result in an incorrect scale. You can only move the entire box, you can not change its dimensions.

If, in the DMC image, the resulting width and height you get after putting in the “resulting resolution” (which is the Cross Track and Along Track enlargement size on the spreadsheet) is less than your desired final plot size, then you can not make a plot of that size at that particular scale.

You can either have a print of a desired area at a desired scale, letting the width and height fall where they may OR you can have a specific scale at a specific width and height.

Any comments or suggestions to improve this method would be appreciated! We think this method maintains the correct scale for plotting. If it does not, why?