Unexpected File Size Reduction
Roger Edrinn
Global Mapper UserTrusted User
Mike,
I opened a number of package files which contained 40' contours along the ocean. All the files were from defective USGS DEMs which had data artifacts along 7.5-min quad borders. We discussed this by email before I joined the forum.
Since I couldn't extract the defective data from the DEMs, I converted them to contours and deleted the defective contours. These defective contours represented less than 1% of the total contours. My process was to open the package file, edit it, then export back to a the same package file name, over writing the original.
The strange thing was that in each case the resultant package file was half the size of its original. I would reopen the new package file and no contours seemed missing. I had simplefication set to zero in the export dialog, no gridding and export all data.
Should I be concerned? How would you account for such a drastic file size reduction?
Thanks
I opened a number of package files which contained 40' contours along the ocean. All the files were from defective USGS DEMs which had data artifacts along 7.5-min quad borders. We discussed this by email before I joined the forum.
Since I couldn't extract the defective data from the DEMs, I converted them to contours and deleted the defective contours. These defective contours represented less than 1% of the total contours. My process was to open the package file, edit it, then export back to a the same package file name, over writing the original.
The strange thing was that in each case the resultant package file was half the size of its original. I would reopen the new package file and no contours seemed missing. I had simplefication set to zero in the export dialog, no gridding and export all data.
Should I be concerned? How would you account for such a drastic file size reduction?
Thanks
Comments
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Roger,
It sounds like your original package file used double-precision coordinates for the contours, while your second export did not (there is a checkbox option for this). Unless you need survey level accuracy (which you likely don't for DEM-based contours of that scale), single precision values will be more than enough, and are only half of the size of the double precision values.
Let me know if I can be of further assistance.
Thanks,
Mike
Global Mapper Support
support@globalmapper.com -
global_mapper wrote: »It sounds like your original package file used double-precision coordinates for the contours, while your second export did not (there is a checkbox option for this). Unless you need survey level accuracy (which you likely don't for DEM-based contours of that scale), single precision values will be more than enough, and are only half of the size of the double precision values.
Since I have many more files from this set, is their a way to open one and determine if it is double-precision?
Thanks
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