Use original ETOPO1 heights and cast ints to float

ETOPO1 is referenced to sea level. I had used a geoid model to convert
it to geometric heights but this is not a great idea. Revert back to the
original. The data were stored as ints to save space but this can cause
problems with divisions and other operations. Cast them back to float
after loading with our functions.

Fixes #29
Fixes #30

data/README.md

@@ -8,13 +8,11 @@ These files are used as sample data in Harmonica:
  geometric (ellipsoidal) height of 10 km. The grid was generated from the EIGEN-6C4
  gravity field model using the [ICGEM Calculation Service](http:https://icgem.gfz-potsdam.de)
  and the WGS84 Reference System. The data are stored in a netCDF file and then `xz`
- compressed using Python's `lzma` library.
+ compressed.
 * `etopo1-0.5deg.nc.xz`: Global relief (topography and bathymetry) of the Earth based on
- the ETOPO1 model at 0.5 degree grid spacing. The original heights referenced to the
- sea level were converted to geometric (ellipsoidal) heights using a geoid grid derived
- from the EIGEN-6C4 gravity field model. Both grids were generated by the
- [ICGEM Calculation Service](http:https://icgem.gfz-potsdam.de). The data are stored in a
- netCDF file and then `xz` compressed using Python's `lzma` library.
+ the ETOPO1 model at 0.5 degree grid spacing. The grid was generated by the [ICGEM
+ Calculation Service](http:https://icgem.gfz-potsdam.de). The data are stored in a netCDF
+ file and then `xz` compressed.
 * `rio-magnetic.csv.xz`: Total-field magnetic anomaly data from the northwestern part of
  an airborne survey of Rio de Janeiro, Brazil, conducted in 1978. Columns are
  longitude, latitude, total field anomaly (nanoTesla), observation height above the

data/examples/gravity_earth.py → data/examples/earth_gravity.py

@@ -1,10 +1,11 @@
 """
-Gravity of the Earth
-====================
+Earth Gravity
+=============
 
 This is the magnitude of the gravity vector of the Earth (gravitational + centrifugal)
-at 10 km height. The data is on a regular grid with 0.5 degree spacing which was
-generated from the spherical harmonic model EIGEN-6C4 [Forste_etal2014]_.
+at 10 km height. The data is on a regular grid with 0.5 degree spacing at 10km
+ellipsoidal height. It was generated from the spherical harmonic model EIGEN-6C4
+[Forste_etal2014]_.
 """
 import matplotlib.pyplot as plt
 import cartopy.crs as ccrs

data/examples/topography_earth.py → data/examples/earth_topography.py

@@ -1,10 +1,11 @@
 """
-Topography of the Earth
-=======================
+Earth Topography
+================
 
 The topography and bathymetry of the Earth according to the ETOPO1 model
 [AmanteEakins2009]_. The original model has 1 arc-minute grid spacing but here we
-downsampled to 0.5 degree grid spacing to save space and download times.
+downsampled to 0.5 degree grid spacing to save space and download times. Heights are
+referenced to sea level.
 """
 import matplotlib.pyplot as plt
 import cartopy.crs as ccrs

harmonica/datasets/registry.txt

@@ -1,3 +1,3 @@
-etopo1-0.5deg.nc.xz 9f21f7c946e649389dce28631de94fa05217750e9ebc77df4311458c33f0682b
+etopo1-0.5deg.nc.xz d975b2f90111043744f70eb1382ed4c654c79065cabe71d067ea8fde7010be2b
 gravity-earth-0.5deg.nc.xz 02c62a251225e2e76722a80d87c488160fbdcae2c1a8bbc2386c32e68ea8f43a
 rio-magnetic.csv.xz 134542c1a0c4b89040a4b7d0e52ff55b97e4a3fb7cbda7004a739166897a5eeb

harmonica/datasets/sample_data.py

@@ -44,7 +44,9 @@ def fetch_gravity_earth():
 
  """
  fname = POOCH.fetch("gravity-earth-0.5deg.nc.xz")
- data = _load_xz_compressed_grid(fname, engine="scipy")
+ # The heights are stored as ints and data as float32 to save space on the data file.
+ # Cast them to float64 to avoid integer division errors.
+ data = _load_xz_compressed_grid(fname, engine="scipy").astype("float64")
  return data
 
 
@@ -56,9 +58,11 @@ def fetch_topography_earth():
  arc-minute grid spacing but here we downsampled to 0.5 degree grid spacing to save
  space and download times. The downsampled grid was generated from a spherical
  harmonic model using the `ICGEM Calculation Service
- <http:https://icgem.gfz-potsdam.de/>`__. See the ``attrs`` attribute of the
+ <http:https://icgem.gfz-potsdam.de/>`__. See the ``attrs`` attribute of the returned
  :class:`xarray.Dataset` for information regarding the grid generation.
 
+ ETOPO1 heights are referenced to "sea level".
+
  If the file isn't already in your data directory, it will be downloaded
  automatically.
 
@@ -69,7 +73,9 @@ def fetch_topography_earth():
 
  """
  fname = POOCH.fetch("etopo1-0.5deg.nc.xz")
- data = _load_xz_compressed_grid(fname, engine="scipy")
+ # The data are stored as int16 to save disk space. Cast them to floats to avoid
+ # integer division problems when processing.
+ data = _load_xz_compressed_grid(fname, engine="scipy").astype("float64")
  return data