[Qgis-user] Imagine raster details in QGIS v ArcGIS

[Nicolas Cadieux]

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] <ml-node+s1560n5283073h24 at n6.nabble.com> a écrit :
> 
> Hi, 
> I downloaded the raster at this site: 
> 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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