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<p>Not a problem:</p>
<p>See below<br>
</p>
<br>
<div class="moz-cite-prefix">On 2018-11-01 4:13 PM, Aj Hollenbach
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CACng8J5dugM5cvqnD+Opn1n8Dg=taXezJ6Z6U2+N4h7dsVxXgA@mail.gmail.com">
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<div>Thanks so much Nicolas---this is extremely helpful! Two
more quick questions, and then I will stop pestering you! :-)</div>
<div><br>
</div>
<div>First, if I change my project back to 4326 now, will that
cause any problems given that I have already loaded 12-15
layers into the project?</div>
</div>
</blockquote>
When In doubt, save the project before making a big change. In this
case, you can move back and forth without trouble.<br>
<blockquote type="cite"
cite="mid:CACng8J5dugM5cvqnD+Opn1n8Dg=taXezJ6Z6U2+N4h7dsVxXgA@mail.gmail.com">
<div dir="ltr">
<div><br>
</div>
<div>Second, are there circumstances in which it would important
to have the project set to a geographically-specific EPSG (and
ensure that all the files are in that same EPSG)? For example,
if you are using an elevation model to delineate a particular
river basin or wanting to measure the acreage of intact forest
in a county in the US, will your results be significantly
different based on whether you have transitioned all of the
files to the "local" EPSG? Basically, I am just trying to get
a feel for when one needs to worry about changing the EPSG.<br>
</div>
</div>
</blockquote>
<br>
Your example is very good. Let's add more info. Imagine the
following:<br>
<ul>
<li>Raster DEM Elevation is in WGS84 Long/lat (epsg 4326) (a
global Geographic CRS). With this, you make a basin file.<br>
</li>
<li>County file is in WGS84/UTM ZONE 20N (in metres) (epsg 32620)
(local CRS)</li>
<li>Your spatial query: What county is in what basin?<br>
</li>
</ul>
<p>If you create the basin in 4326 you need to know exactly how the
spatial query will be made by the software. If there is no
reprojection, files will not align and the query does not work.
If the software (or plugin) does a reprojection, you need to know
what algorithm will be used and trust it. Results can change
according to the method used. See the documentation with the
plugins or contact the person responsible.<br>
</p>
The other way is to do all the work yourself so you don't have to
trust a coder. You could<br>
<ul>
<li>Reproject the Raster from 4326 to 32620 then make the vector
basin file or</li>
<li>Make the vector basin file in 4326 then reproject that in
32620. This is probably better and will lead to better results
as reprojecting a raster does change the raster a lot depending
on the algorithm chosen (nearest neighbour, bilinear, cubic...)
( <a class="moz-txt-link-freetext" href="https://www.gdal.org/gdalwarp.html">https://www.gdal.org/gdalwarp.html</a>)</li>
</ul>
Basically, you need to evaluate the impact of the reprojection on
each data source and make the best decision... or trust that the
software is doing a good job. If your research depends on it, you
better do it yourself.<br>
<br>
Nicolas<br>
<br>
<blockquote type="cite"
cite="mid:CACng8J5dugM5cvqnD+Opn1n8Dg=taXezJ6Z6U2+N4h7dsVxXgA@mail.gmail.com">
<div dir="ltr">
<div><br>
</div>
<div>Thanks Again,</div>
<div>Allen <br>
</div>
<div><br>
<div class="gmail_quote">
<div dir="ltr">On Thu, Nov 1, 2018 at 2:45 PM Nicolas
Cadieux <<a href="mailto:nicolas.cadieux@archeotec.ca"
moz-do-not-send="true">nicolas.cadieux@archeotec.ca</a>>
wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<div text="#000000" bgcolor="#FFFFFF">
<p>Hi,</p>
<p>Unless you have a specific problem with the project
being in a certain CRS like 3125, there is no need to
change that in the project. If Project on the fly is
working with every layer, all is good. If your
project is in 3125, have one file in 3125 and 10 in
4326, then you need to know that QGIS is reprojecting
10 files to 3125. That takes time especially if you
have very big .shp files or rasters. In that case, it
may be faster to change the project in 4326 to speed
rendering up. Occasionally, you will have plugins
that do not work unless all the files and the project
have the same CRS. Keep that in mind.</p>
<p>Apart from that, it comes down to the look and feel
of the map and even that can now be changed in
printing layout (since Qgis 3 I think). <br>
</p>
<p>Cheers!<br>
</p>
<p>Nicolas<br>
</p>
<br>
<div class="m_6449513910343160932moz-cite-prefix">On
2018-11-01 2:26 PM, Aj Hollenbach wrote:<br>
</div>
<blockquote type="cite">
<div dir="ltr">
<div>Thanks much for your advice Nicolas. I have
indeed been using a basemap (google satellite) and
all layers are well georeferenced even though I
have made no changes to the files themselves in
terms of changing the CRS. The only changes that I
have made have made are at the project level,
whereby I have assigned the project a CRS of EPSG
3125. Are you recommending that I just change the
project-level CRS back to 4326? Put differently,
is there any advantage to using EPSG 3125?</div>
<div><br>
</div>
<div>Thanks Much,</div>
<div>Allen<br>
</div>
</div>
<br>
<div class="gmail_quote">
<div dir="ltr">On Thu, Nov 1, 2018 at 1:57 PM
Nicolas Cadieux <<a
href="mailto:nicolas.cadieux@archeotec.ca"
target="_blank" moz-do-not-send="true">nicolas.cadieux@archeotec.ca</a>>
wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<div text="#000000" bgcolor="#FFFFFF">
<p>Hi,</p>
<p>These are two very different projections. I
would load a base map from Quickmap services
plugin to see if files are being loaded
correctly. If the files are in 4326, and are
loaded in 4326, then all is good if they are
well georeferenced (use the Quickmap base map
to make sure). If files don't have a CRS,
they may been incorrectly loaded if you have
the "use project CRS" or "Use a default CRS"
in the options/CRS/CRS for new layers menu.</p>
<p>If a file is in 4326 but incorrectly loaded
in another projection, you need to correct the
CRS not reproject it with GDAL WARP. Simply
right click on the layer in the layers panel,
change the CRS (Set CRS/set layer CRS) (Qgis
3x) and save the file under a new name. That
should change the CRS and the file position on
the map. If you are still in the wrong place,
you need to make some guess work.<br>
</p>
<p>Nicolas<br>
</p>
<p
style="margin-top:0px;margin-bottom:0px;margin-left:0px;margin-right:0px;text-indent:0px"><br>
</p>
<p
style="margin-top:0px;margin-bottom:0px;margin-left:0px;margin-right:0px;text-indent:0px">Extent:
-180.00, -90.00, 180.00, 90.00 Proj4:
+proj=longlat +datum=WGS84 +no_defs</p>
<p
style="margin-top:0px;margin-bottom:0px;margin-left:0px;margin-right:0px;text-indent:0px"><br>
</p>
<p
style="margin-top:0px;margin-bottom:0px;margin-left:0px;margin-right:0px;text-indent:0px">Extent:
123.73, 4.76, 126.65, 21.97 Proj4: +proj=tmerc
+lat_0=0 +lon_0=125 +k=0.99995 +x_0=500000
+y_0=0 +ellps=clrk66
+towgs84=-127.62,-67.24,-47.04,-3.068,4.903,1.578,-1.06
+units=m +no_defs</p>
<br>
<div
class="m_6449513910343160932m_-4461126888258208994moz-cite-prefix">On
2018-11-01 1:02 PM, Aj Hollenbach wrote:<br>
</div>
<blockquote type="cite">
<div dir="ltr">
<div>Hi QGIS Community,</div>
<div><br>
</div>
<div>I have what I believe may be a fairly
straight-forward question on coordinate
reference systems and projections in QGIS.
I am currently working on some maps in the
southern Philippines and thus have set my
"project-level" coordinate reference
system (CRS) to EPSG 3125. I have noticed,
however, that when I open a vector file or
raster file that is loaded into this
project, the CRS still reads as EPSG 4326.
Thus, my question is whether it is
necessary to "manually" go in and change
the EPSG of each new layer to EPSG 3125
(i.e. use the Warp Tool for a raster file
or the "Reproject Layer" tool for a vector
file), or if there is any issue with
leaving them as EPSG 4326. I note that I
am not having any issues "seeing" the
newly added files--meaning the
"on-the-fly" projection seems to be
working fine. Appreciate any insights that
you might have.</div>
<div><br>
</div>
<div>Regards,</div>
<div>Allen<br>
</div>
</div>
<br>
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