Hello Tony,
I second David's suggestions. If you'd like some additional material to
refer to, here's the QGIS workshop manual I use for my day-long sessions.
Chapter 4 includes a discussion of spatial / coordinate reference systems:
https://libguides.brown.edu/gis_data_tutorials/intro_qgis
I always advise students that they should use a system that fits their
intended purpose, and the number of choices they'll have varies with scale.
At a global level you have a lot of choices, but they get narrower as you
drill down. Each continent and many large countries have conformal, equal
area, and equidistant projections (Lambert, Albers, etc) that you can
choose from based on the property you want to preserve. Many mid-size to
smaller countries will have their own preferred systems, or you can choose
a relevant UTM zone. For smaller areas, you can also choose a relevant UTM
zone, or in the US the state plane system for the area.
In terms of how the transformations work... I simply tell folks to think of
these as reference systems or standards for modeling the Earth. Each system
has three components: an approximation of the earth's shape (spheroid /
ellipsoid), a datum (math for attaching the spheroid / ellipsoid to the
true shape of the earth), a coordinate system, and a map projection.
Systems with the first 3 components are geographic coordinate systems (GCS)
for modeling things on a three dimensional sphere. Systems with all 4
components are projected coordinate systems (PCS) for modeling things in
two dimensions. QGIS has a library of all these standards, so you can
convert your data from one to the other based on your intended purpose. For
most applications, I suggest that people convert their data from a GCS
(like WGS 84 or NAD 83) to an appropriate PCS. And I leave it at that!
Best - Frank
On Thu, May 19, 2022 at 12:34 PM David Medeiros <[log in to unmask]>
wrote:
> Sorry Tony, I read your question twice and somehow still missed that you
> were talking about a full class and not a workshop! Not sure anything I
> sent just now will apply to what you need. That said, I would still not put
> too much time into explaining transformations other than to describe them
> as the crosswalk between coordinate systems that are based on different
> datums. The fine details of dealing with transformations in a practical
> sense are more black-art than anything IMO and hard to teach theoretically.
> Most GIS users at research institutions don't need that level of
> understanding.
>
> David
>
> *David Medeiros*
>
> Geospatial Reference & Instruction Specialist
>
> Stanford Geospatial Center
>
> 650.561.5294
>
> @mapbliss
>
>
> SGC website: gis.stanford.edu
>
> GIS cartography: bit.ly/giscart <http://bit.ly/giscart>
> <http://bit.ly/giscart>
>
> GIS email list: bit.ly/GISlist
>
>
> ------------------------------
> *From:* Maps-L: Map Librarians, etc. <[log in to unmask]> on behalf
> of Diaz, Tony A. <[log in to unmask]>
> *Sent:* Thursday, May 19, 2022 7:31 AM
> *To:* [log in to unmask] <[log in to unmask]>
> *Subject:* QGIS, question about projection transformation screen
>
>
> Hi folks, I’m going to start teaching an introduction to QGIS class later
> this year, for newly arriving students here at my institute.
>
>
>
> Would anyone have an easy explanation I can say about projection
> transformations? I have an idea of what it is, but I was hoping if someone
> could chime in on a simple explanation for someone new to GIS to understand
> what this means when a user comes across it.
>
>
>
> Also, what transformation would one use and why? (Example, in the
> attached screenshot, which of these would I choose and why).
>
>
>
>
>
> Thanks again,
>
>
>
> Tony Diaz
>
> California Institute of Technology
>
> Pasadena, CA
>
--
Frank Donnelly (he/him/his)
GIS & Data Librarian | Brown University
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https://libguides.brown.edu/gis | https://atcoordinates.info/
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