Mike and Nicolas,
Thanks for taking the time to explain all of this to me, it did clear up
some misunderstandings I had.
best regards,
John
On 8/30/2016 12:54 PM, Michael Treglia wrote:
Hey John,
I've dealt with this layer a bit too - one thing for QGIS is that when
you import a layer, I think the default to calculate the min/max based
on the 2-98% range of the data. If you're in the Layer Properties ->
Style tab, expand the 'Load min/max values' area, and then select 'Min
/ max' (and change Accuracy to 'Actual'), and then hit the load
button. That yields the range as 207-15516. (see screen-shot below). I
know this is a categorical raster, so displaying based on the color
ramp is meaningless, but that at least shows you're getting the right
values.
Re the projection - I thought this used to display correctly based on
the projection parameters that get read in, but it's displaying way
far north. Toying around, it looks like the equivalent EPSG code is
5070 (+proj=aea +lat_1=29.5 +lat_2=45.5 +lat_0=23 +lon_0=-96 +x_0=0
+y_0=0 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs)
FWIW, I did this on QGIS 2.16.1
Hope that helps,
mike
Inline image 1
On Tue, Aug 30, 2016 at 12:23 PM, Nicolas Cadieux
<[email protected] <mailto:[email protected]>>
wrote:
Hi,
The easy answer is no:(
That could happen If the nature of the file was different (like a
simple picture .jpg) but in this case, as in most, you have
choices to make and assumptions are made by the programs based
partly on the user settings. The problem is that most rasters used
in GIS have more than the usual 256 values you find in a picture.
Therefore, you need to select what part of the image you want to
display or render on the screen which is limited to 256 values only.
As an example, you can ask QGIS to automatically open a raster
using the min-max pixel or you can ask Qgis to cut off the higher
and lower 1%. (Also, min max values can be real (slow) or
estimated (fast)). The second option may be better if you know you
have noise in the data. To have a true idea of what the real min
max values are, you need to run the statistics. You can't rely on
rendering values shown under the layers.
The problem you are having with the CRS is probably that ArcGis or
ERDAS, is coding the information differently than in Qgis. Some
software companies do it differently either because they want to
lock you in a proprietary format or because they think their
format is better. If I recall, ERDAS does not put the
information in the .tiff but rather, in a text file. (I may be
wrong here.) In any case, look an compare the CRS definitions.
They maybe identical. You can also click on the layer and specify
the CRS. If you want to change the CRS, you will need to use the
save as option.
ArcGIS also has a concepts of dealing with rasters that have
unique values. The idea, is that you can have a raster with a
data base. After that, pixels with the value of 1, for example,
could be identified as "Roads". That concept does not exist in
Qgis (as of 2.12). You can however, make a colour palette for the
unique values but Qgis will not id the values as objects like
roads or building. I imagine this will come soon if it's not
already in 2.16.
Unfortunately, all those choices used for rendering get saved in
the project files and while most GIS will open a great number of
file formats correctly, most are completely incapable of reading
each other's project files:(. If they can, they have very limited
options.
Hope this clarifies things a bit.
Nicolas
Envoyé de mon iPad
Le 30 août 2016 à 11:24, john.polo [via OSGeo.org
<http://osgeo.org>] <[hidden email]
<http:///user/SendEmail.jtp?type=node&node=5283092&i=0>> a écrit :
Hi,
I downloaded the raster at this site:
http://www.wildlifedepartment.com/facts_maps/ecoregions.htm
<http://www.wildlifedepartment.com/facts_maps/ecoregions.htm>
(scroll down about halfway to find the raster link. The file is
kind of
big, ArcGIS says the uncompressed size is 5.69 GB, 225 MB as a zip)
and loaded it into QGIS as a standalone raster file. It is a
raster of
Oklahoma, USA classified into vegetation types at 10mx10m
resolution.
It is saved as ERDAS Imagine, if I understand the extension. I don't
know if the file type is relevant to the issues I am running into.
When the file is open in QGIS, the min and max pixel values have a
smaller range, 314 - 14797, than when the file is open in ArcGIS,
207 -
15516. I can redraw the raster in ArcGIS to "Unique" instead of
"Stretched" pixel values in the Symbology, which will then give me
classifications for the pixels. Additionally, the CRS is
USER:100002 in
QGIS, but Albers Conical Equal Area in QGIS. Why does this
difference in
rendering happen? How can I get QGIS to render the file with the
same
characteristics as ArcGIS automatically? I know I can save with a
new
CRS to fix that issue, but if the file can load in ArcGIS with
the CRS
from the start, shouldn't QGIS render it the same? I don't know
how to
get the pixel values to the same range or how to get the
classifications
assigned.
John
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