A post about the MGRS and UTM grids

[florisvdh]

Description

Finally. This blog post for the tutorials website reports findings from a 'quest', which started several months ago when @ToonHub asked me to review a PR that used the MGRS grid (at that time called UTM grid). Having used that grid myself a long time ago in nature inventories, just like many other people, I was (inevitably) puzzled to find out about its foundations and authoritative definition, its relation to coordinate reference systems, and where it is being maintained (this appeared not all too obvious at first...). All that, and more, is now explained in this post – that's the intention at least. Intermediate findings, prior to this writing-up, have also been discussed with NGI, @carinewils and @wlangera.

Standalone files that can also be used for reviewing and to provide comments:

• index.html.zip
• index.pdf
• index.docx

Previewing the pull request

Thanks to GitHub Actions, an artifact (=zip file) of the rendered website is automatically created for each pull request. This provides a way to preview how these updates will look on the website, useful to contributors and reviewers.

Instructions to preview the updated website

1) On the PR page, you can find a "details" link under "checks - On PR, build the site and ...". Go there, click on the top link in the left sidebar ("Summary"), and download the generated artifact at the bottom of the page. 2) Decompress it and make sure the target directory is called 'tutorials' (you may need to rename it) 3) From the parent directory (just above the tutorials folder you created/renamed), run python -m http.server 8887, or launch the Google Chrome Web Server app and point it at the parent directory. 4) Point your browser to http://localhost:8887/tutorials. 5) Review the updated website. As a contributor, you can push extra commits to update the PR. As a reviewer, you can accept/refuse/comment the PR.

Note: for step 3, you can use any other simple HTTP server to serve the current directory if you don't have a Python 3 environment or Google Chrome available.

[ToonHub]

Nice work!

I suggest to make the summary as simple as possible and avoid technical issues that are not essential for your take home messages. This way readers with limited technical background can grasp the messages.

Therefore I suggest to remove following sentences from the summary and move them to the introduction

Authorative definitions of widely used grids and grid reference systems are not globally coordinated though, contrary to the case of the EPSG-dataset for coordinate reference systems.

The full definitions are not given here since the primary aim is to draw attention to important properties and differences. For the same reason, the UPS portion of MGRS is not further explained here.

This post results from a limited literature study on these topics (see Bibliography).

In the summary I would add an example of both coding systems in the sentence below. This way readers can recognize more easily which grid they are using.

UTM grid references consist solely of digits (Arabic numerals), while MGRS grid references are alphanumeric (combination of digits & Latin characters).

If I am not mistaken grid cells with a certain precision overlap for both grid types. Only the coding is different. Maybe this can be emphesized.

[florisvdh]

Thanks for reviewing and for these suggestions @ToonHub; I think I can implement these or most of them!

If I am not mistaken grid cells with a certain precision overlap for both grid types. Only the coding is different. Maybe this can be emphesized.

It depends on the cells you're looking at, but maybe that's what you mean by 'certain precision'. In MGRS, the UTM grid cell pattern is intersected by MGRS grid zone borders (see the two bullets under MGRS). So you get cell splits there, effectively yielding more cells than in the UTM grid, typically along the bordering parallels. In some places the meridian borders are shifted too compared to the UTM zones, which raises more differences. Only "below the grid zone level it is still the UTM grid pattern that drives the further subdivision". At that level you are right, and this could indeed be stressed more explicitely, perhaps repeated in the summary.

[florisvdh]

@hansvancalster @ToonHub @wlangera I finished processing your comments, and added or updated more things. Thanks for the fruitful reviews!

The standalone files linked at the top of this pull request have been updated.

[florisvdh]

The standalone files linked at the top of this pull request have been updated.

[florisvdh]

Thank you for reviewing @amber-mertens.

Indeed, I haven't paid attention to controlling PDF page layout, where figures are automatically moved to separate pages. In HTML this is not a problem so will leave it as-is.