[Qgis-user] 1928-1930 USGS maps

[William Kyngesburye]

Max 1?  Odd for a greyscale image.  Or are they 1bit images?

At least it's working.

> On Dec 28, 2015, at 4:12 PM, Jan Becket <janbecket.net at gmail.com> wrote:
> 
> With your suggestions and those of Nicolas I was able to make the maps appear. I trashed the copies I had altered in Photoshop, loaded the original (black) images, set the min as 0 and the max as 1 and then stretched to min/max. That turned out to be easy. Thanks for the help!
> 
> - Jan
> 
> 
>> On Dec 28, 2015, at 4:42 AM, William Kyngesburye <woklist at kyngchaos.com> wrote:
>> 
>> Or, QGIS should be able to read 16bit greyscale images without breaking them with Photoshop.  You just need to set a min and max to include the 16bit range and stretch the contrast to minmax.  Make sure you are not using QGIS 2.10 - that had a broken raster styling on OS X.
>> 
>> Alternately, use GDAL to convert to 8bit, not Photoshop.  "gdal_translate -scale -ot Byte" should do it (in Terminal, I don't see the -scale or -ot options in QGIS GDAL convert format).
>> 
>>> On Dec 28, 2015, at 12:53 AM, Nicolas Cadieux <nicolas.cadieux at archeotec.ca> wrote:
>>> 
>>> Hi,
>>> You are correct, you have stripes the CRS info from the map because Photoshop does not know what to do. If you load the map, you can click on raster.. Something... Gdal-info.  That should give you all the tagged info left in the image.
>>> 
>>> Load the map in QGIS and click on "zoom to layer extent".  If you still don't see it, make sure you are in a CRS that is projected in meters.  You should be able to see it.  The map should be at 0,0.
>>> 
>>> All is not loss...  You can reproject the map to the correct CRS using the gdal georeferencing tool.    Always back up your data before manipulating it.
>>> Hope this helps!
>>> 
>>> Nicolas Cadieux M.Sc.
>>> Les Entreprises Archéotec inc. 
>>> 8548, rue Saint-Denis Montréal H2P 2H2
>>> Téléphone: 514.381.5112  Fax: 514.381.4995
>>> www.archeotec.ca
>>> 
>>> On Dec 28, 2015 01:16, "Jan Becket [via OSGeo.org]" <[hidden email]> wrote:
>>> After much effort, I obtained an old set of 1928-1930 USGS topo maps of Oʻahu Island, Hawaiʻi - for a project I am working on. The maps are without collars and used by a local archaeological firm. The geotiff maps at first showed as completely black on my Mac system (10.10.5 - Yosemite). However I found this link with a resultuion for the issue:
>>> 
>>> http://stackoverflow.com/questions/13925995/options-to-convert-16-bit-image
>>> 
>>> I opened the 16 quads for Oʻahu in Photoshop and applied auto levels, after which they became discernable quad maps. However, when imported into QGIS, they are now invisible. I wonder if it is possible that I stripped away the geotiff CRS information. Prior to the Photoshop operation, the maps did appear correctly referenced to Oʻahu Island (although they were totally opaque / black).
>>> 
>>> Now, the maps in QGIS are not visible, although the geotiffs are several megabytes in size and are visible when opened in Photoshop, Preview, or whatever image viewer. 
>>> 
>>> I should mention that I have successfully imported other sets of USGS maps of Oʻahu for this project - the 1902 series, the 1916 series, the 1935 series - as well as the current series. The early maps used the Old Hawaiian CRS (EPSG 3564) but more modern maps use WGS83, Zone 4N (EPSG 3711). I have on the fly projection enabled and have tried many CRS settings for the maps, but nothing results in a visible quad appearing.
>>> 
>>> Suggestions very welcome ...

-----
William Kyngesburye <kyngchaos*at*kyngchaos*dot*com>
http://www.kyngchaos.com/

"This is a question about the past, is it? ... How can I tell that the past isn't a fiction designed to account for the discrepancy between my immediate physical sensations and my state of mind?"

- The Ruler of the Universe