Add information to Quickstart on basics of getting measurement values and navigation

doc/source/quickstart.rst

@@ -130,6 +130,22 @@ To access the loaded data use the wavelength or name:
 For more information on loading datasets by resolution, calibration, or other
 advanced loading methods see the :doc:`readers` documentation.
 
+
+Calculating measurement values and navigation coordinates
+=========================================================
+
+Once loaded, measurement values can be calculated from a DataArray within a scene, using .values to get a fully calculated numpy array:
+
+    >>> vis006 = global_scene["VIS006"]
+    >>> vis006_meas = vis006.values
+
+Note that for very large images, such as half-kilometer geostationary imagery, calculated measurement arrays may require multiple gigabytes of memory; using deferred computation and/or subsetting of datasets may be preferred in such cases.
+
+The 'area' attribute of the DataArray, if present, can be converted to latitude and longitude arrays. For some instruments (typically polar-orbiters), the get_lonlats() may result in arrays needing an additional .compute() or .values extraction.
+
+    >>> vis006_lon, vis006_lat = vis006.attrs['area'].get_lonlats()
+
+
 Visualizing data                                                                                    
 ================                                                                                    
 
@@ -246,6 +262,26 @@ output format and to provide the best looking image. For more information
 on saving datasets and customizing enhancements see the documentation on
 :doc:`writers`.
 
+
+Slicing and subsetting scenes
+=============================
+
+Array slicing can be done at the scene level in order to get subsets with consistent navigation throughout. Note that this does not take into account scenes that may include channels at multiple resolutions, i.e. index slicing does not account for dataset spatial resolution.
+
+  >>> scene_slice = global_scene[2000:2004, 2000:2004]
+  >>> vis006_slice = scene_slice['VIS006']
+  >>> vis006_slice_meas = vis006_slice.values
+  >>> vis006_slice_lon, vis006_slice_lat = vis006_slice.attrs['area'].get_lonlats()
+
+To subset multi-resolution data consistently, use the :meth:`~satpy.scene.Scene.crop` method.
+
+  >>> scene_llbox = global_scene.crop(ll_bbox=(-4.0, -3.9, 3.9, 4.0))
+  >>> vis006_llbox = scene_llbox['VIS006']
+  >>> vis006_llbox_meas = vis006_llbox.values
+  >>> vis006_llbox_lon, vis006_llbox_lat = vis006_llbox.attrs['area'].get_lonlats()
+
+
+
 Troubleshooting
 ===============