Lookup altitude

[nicomt]

• [x] Closes #1516
• [x] I am familiar with the contributing guidelines
• [x] Tests added
• [x] Updates entries in docs/sphinx/source/reference for API changes.
• [x] Adds description and name entries in the appropriate "what's new" file in docs/sphinx/source/whatsnew for all changes. Includes link to the GitHub Issue with :issue:`1516` or this Pull Request with :pull:`1518`. Includes contributor name and/or GitHub username (link with :ghuser:`nicomt`).
• [x] New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
• [x] Pull request is nearly complete and ready for detailed review.
• [ ] Maintainer: Appropriate GitHub Labels (including remote-data) and Milestone are assigned to the Pull Request and linked Issue.

To make it easier to review, this pull request doesn't replace the default altitude in the Location class or the get_solarposition yet. I will make another pull request once this one is merged. One change to note is _degrees_to_index; I simplified the function. I think a lot of the centering and floating point logic is unnecessary. I even compared the output over several thousands of random integers, and their outputs were identical. Let me know if you rather I change it back.

[nicomt]

FYI, I want to be more transparent about how I generated the altitude dataset. I'm trying to consolidate the generation into one python script so is easy for you guys to run it. I will probably release it as a Github Gist soon.

Also, I want to do a better job quantifying the accuracy of the generated map. I will try to find a good source of ground truth for altitudes and release some error stats.

[nicomt]

Here is the Gist I promised that allows you guys to reproduce the altitude map. https://gist.github.com/nicomt/d2c5f08a8ee3500550be42d8dbee6c1d

While compiling the script, I realized that the lower altitude bound in my first commit was lower than the lowest place on earth (The Death Sea). In this version, I fixed the lower bound to -450m, which removes those bad values and allows me to reduce the step 28m instead of 35m.

Also, I compared the results with a few samples from the Google Elevation API, and here is what I got.

It looks like the error is pretty bad in Greenland and Asia but relatively okay elsewhere. Here are the actual results of my test.

Metric	Error
Mean Absolute Error	113
Max Error	3067
Mean Squared Error	133069

Let me know if there is something else I can do to help.

[mikofski]

Thanks so much Nicolas, Two very minor nits that aren't blockers and can be improvements in future PR's, I don't want to hold up merging this asap.

1. in test_lookup_altitude() a fixture might work better instead of looping over test_locations
2. in location.lookup_altitude() it would be nice if we could adopt pathlib.Path and the / operator for all new code and retire osp.join(). EG:

   import pathlib
   pvlib_path = pathlib.Path(__file__).parent
   filepath = pvlib_path / 'data' / 'Altitude.h5'
   

[nicomt]

@mikofski I will fix those in the next PR. Thanks