[Qgis-user] Imagine raster details in QGIS v ArcGIS
john polo
jpolo at mail.usf.edu
Tue Aug 30 11:12:32 PDT 2016
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
> <nicolas.cadieux at archeotec.ca <mailto:nicolas.cadieux at archeotec.ca>>
> 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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