Global Mapper v25.0

Unexpected File Size Reduction

Roger Edrinn
Roger Edrinn Global Mapper UserTrusted User
edited January 2008 in Technical Support
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

Comments

  • global_mapper
    global_mapper Administrator
    edited January 2008
    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
  • Roger Edrinn
    Roger Edrinn Global Mapper User Trusted User
    edited January 2008
    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.
    I would not have purposely selected double-precision. However, I could easily have done so accidently. Many switches in the dialog, so very possible. Certainly a plausible explanation.

    Since I have many more files from this set, is their a way to open one and determine if it is double-precision?

    Thanks