Importing my own map sheets results in poor resolution due to scaling.
I'm trying to generate highly detailed 1:12000 8.5x11 or 11x17 map & compass exercise maps that I use to run local workshops.
To do this, I've exported the topo map layer from the latest 2023 USGS pdf maps at high resolution (400 or 600 dpi) to PNG and tried to import them to Caltopo and using hand alignment.
I can align the map layer just fine, but the resulting MapSheet layer's resolution is nowhere near the original imported file. It's 200 DPI at best. Contour lines are jagged, and tick-marks for depressions (common in our area) are blurry and hard to see when printed, even on a nice printer at Kinko's.
I highly suspect that Caltopo is down-sampling the resolution of my imported PNG file to save server storage space. Is there any way to prevent this?
My only course of action at the moment is to create my own maps by hand in a photo-editing tool, re-adding newer trails, etc, and this is what I was counting on Caltopo to do for me.
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We'd be happy to take a look at these specific mapsheets with you, please send us an email at help@caltopo.com, and include the email associated with your account so we can pull them up.
Same experience, categorically. Giving Caltopo the benefit of the doubt, some resampling may be necessary in order to handle the map alignment process on a pixel level.
I want to illustrate that this is an issue even for map sheets that aren't very high resolution. For instance, here is a map sheet I'd like to upload, available here: https://www.adfg.alaska.gov/static/fishing/PDFs/commercial/kodiak/kodiak_salmon_statisticalareas.pdf.
Before georeferencing, a portion of it looks like this:
This is a zoomed-in portion of a non-georeferenced PDF that's about 7MB, and only about 72 dpi. But after uploading and georeferencing, the same region looks like this:
... the effect being that the map is almost unusable (labels are unreadable).
There must be a workaround for this, because some shared map sheets already available on CalTopo (for example the North Cascades National Park Backcountry Map) maintain the high resolution of the original source, even though the original source is distributed as a non-geo PDF. As an experiment, I tried to upload and georeference the same NPS-sourced pdf myself, but the result is again very low-resolution and barely usable compared to the version already available as a shared CalTopo map sheet.
I understand the point about pixel resampling for map alignment, but such a drastic reduction in quality doesn't occur when georeferencing the same images in e.g. QGIS (which perhaps is the best workaround). It seems that for user-uploaded map sheets to be useful, this needs to be fixed. Alternatively, it would be helpful to know a workaround by which we can upload and geo reference map sheets and maintain at least usable resolution.
As always, despite my minor gripes like this one, I'm thankful to have such a wonderful mapping product!
Is there any update to this issue? I am struggling with map sheet resolution as well. It seems that when I am aligning the map it is in the same resolution as the native file but when I save it renders it in a lower resolution. Quite annoying.
Please email us with the file and your account. This is something we need to look at with you specifically. help@caltopo.com
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