[Qgis-user] Creating Geological Maps

Tue Oct 18 04:53:51 PDT 2011

Hi Gerhardus,
        Just to clarify on image file formats a little:

JPEG - Best used with photos. You're correct that it loses quality when 
compressed. This is called "lossy compression".

PNG - Also commonly used with photos, the compression with this is 
lossless, so you can set it as high as you like and no quality will be 
lost.

Generally speaking while good for storing general imagery, the above two 
formats don't typically hold spatial information so aren't used in a GIS.

TIF (also TIFF) - There's about a trillion different types of TIF file 
format features. This is the file type that in the GIS world is commonly 
used to store raster imagery. There are others, but this one is the most 
supported, even if different aspects are supported by different 
applications.
By default they're uncompressed, but they can be compressed using a number 
of methods, LZW compression is a popular choice and is lossless. TIFs 
themselves don't always store spatial information but may come with a 
"world file" (tfw or tab format) which is a small text file which contains 
that information. Some TIFs do store spatial information within the TIF 
file itself and these are called "GeoTIFFs".
Some (geo)tiffs can also store "pyramids" - these make large rasters 
display faster when zoomed out.

Hopefully that clears things up a little.
Regards,

Jonathan

From:   Gerhardus Geldenhuis <gerhardus.geldenhuis at gmail.com>
To:     tech at wildintellect.com
Cc:     qgis-user at lists.osgeo.org
Date:   16/10/2011 22:26
Subject:        Re: [Qgis-user] Creating Geological Maps
Sent by:        qgis-user-bounces at lists.osgeo.org

On Sun, Oct 16, 2011 at 10:03 PM, Alex Mandel <tech_dev at wildintellect.com> 
wrote:
On 10/16/2011 01:32 PM, Gerhardus Geldenhuis wrote:
Just to check, do you have projection on-the-fly turned on? For best
image quality the image needs to be in it's native projection. Though in
this case it sounds like you just opened the image file to look at it.
How does it look in something like GIMP when you zoom in?

Hi Alex
Many thanks for the reply!

No idea if I have project on-the-fly turned on, I could not see any 
reference to it in the project properties or options.
Native projection? I scanned the paper images and the scanner saved them 
as jpeg files. I then opened up the images in The Gimp rotated 
and aligned the to a drawing grid in Gimp because they were about 1 degree 
skewed. I had pencilled lines on the paper that represented UTM grid 
lines. I saved these as both jpg and png format both in lowest compression 
thus highest quality. Viewing either the jpg or png file in Gimp or Image 
Viewer is fine and if I zoom in to 1:1 scale the quality is perfect.

> Second question is how can I "align" this png file with with an actual
> co-ordinate system. I have marked 4 points on my paper maps and lines
> between these points that represent the UTM grid on which I worked and I
> would need to at least roughly align this image with the UTM grid.
>

Georeferencing plugin, this will turn your tiff into a GeoTiff, the same
as what you might get from preprocessed ASTER data. You can mark and
enter known coordinate locations and then warp the image to fit.

That sounds excellent I am going to try that now.

> Thirdly I am not sure how to draw on a new layer but still view my image 
as
> a template for the digitizing.
>
> I am more than happy to read any documentation and work through 
tutorials.
> What is lacking at the moment is the terminology to search for the 
correct
> things and experience. I am working through a number of tutorials but 
are
> still struggling a bit to connect A to B.
>
Read the manual on the QGIS website, create a new vector layer and
editing it can be over anything else you have loaded in the map view.
http://qgis.org/en/documentation/manuals.html

> I will eventually add some ASTER imagery but it is my understanding that
> these are in tiff format and contains extra information like spatial
> referencing. I suspect that if I can master the above then adding ASTER
> imagery will be easy.
>
> Lastly is there any geological map symbol databases available?
>
It's been discussed and some work has been done on it but nothing
comprehensive yet. If you have Images or Fonts of the symbols you want
you can easily convert them for use in QGIS.

A shame, I have send of an email to the British Geological Survey and see 
what they come back with. I might have access to the ArcGIS and could load 
the images database provided by the US Geological Survey. Not sure if 
ArcGIS would allow me to export it into something more usable. My google 
results turned up an email thread which suggested that there was some work 
done on supporting gdb files... 

Regards

-- 
Gerhardus Geldenhuis_______________________________________________
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