Other data sources

If you want to run your own workflow from A to Z, or if you would like to know which data are used in OGGM before being available in the pre-processed directories, read further for some more details!

Glacier outlines and intersects

Glacier outlines are obtained from the Randolph Glacier Inventory (RGI). We recommend to download them right away by opening a python interpreter and type:

from oggm import cfg, utils
cfg.initialize()
utils.get_rgi_intersects_dir()
utils.get_rgi_dir()

The RGI folders should now contain the glacier outlines in the shapefile format, a format widely used in GIS applications. These files can be read by several software (e.g. qgis), and OGGM can read them too.

The “RGI Intersects” shapefiles contain the locations of the ice divides (intersections between neighboring glaciers). OGGM can make use of them to determine which bed shape should be used (rectangular or parabolic). See the rgi tools documentation for more information about the intersects.

The following table summarizes the RGI attributes used by OGGM. It can be useful to refer to this list if you use your own glacier outlines with OGGM.

RGI attribute	Equivalent OGGM variable	Comments
RGIId	gdir.rgi_id	[1]
GLIMSId	gdir.glims_id	not used
CenLon	gdir.cenlon	[2]
CenLat	gdir.cenlat	[2]
O1Region	gdir.rgi_region	not used
O2Region	gdir.rgi_subregion	not used
Name	gdir.name	used for graphics only
BgnDate	gdir.rgi_date	[3]
Form	gdir.glacier_type	[4]
TermType	gdir.terminus_type	[5]
Status	gdir.status	[6]
Area	gdir.rgi_area_km2	[7]
Zmin	glacier_statistics.csv	recomputed by OGGM
Zmax	glacier_statistics.csv	recomputed by OGGM
Zmed	glacier_statistics.csv	recomputed by OGGM
Slope	glacier_statistics.csv	recomputed by OGGM
Aspect	glacier_statistics.csv	recomputed by OGGM
Lmax	glacier_statistics.csv	recomputed by OGGM
Connect	not included
Surging	not included
Linkages	not included
EndDate	not included

For Greenland and Antarctica peripheral glaciers, OGGM does not take into account the connectivity level between the Glaciers and the Ice sheets. We recommend to the users to think about this before they run the task: workflow.init_glacier_directories.

Comments

[1]

The RGI id needs to be unique for each entity. It should resemble the RGI, but can have longer ids. Here are example of valid IDs: RGI60-11.00897, RGI60-11.00897a, RGI60-11.00897_d01.

[2] (1,2)

CenLon and CenLat are used to center the glacier local map and DEM.

[3]

The date is the acquisition year, stored as an integer.

[4]

Glacier type: 'Glacier', 'Ice cap', 'Perennial snowfield', 'Seasonal snowfield', 'Not assigned'. Ice caps are treated differently than glaciers in OGGM: we force use a single flowline instead of multiple ones.

[5]

Terminus type: 'Land-terminating', 'Marine-terminating', 'Lake-terminating', 'Dry calving', 'Regenerated', 'Shelf-terminating', 'Not assigned'. Marine and Lake terminating are classified as “tidewater” in OGGM and cannot advance - they “calve” instead, using a very simple parameterization.

[6]

Glacier status: 'Glacier or ice cap', 'Glacier complex', 'Nominal glacier', 'Not assigned'. Nominal glaciers fail at the “Glacier Mask” processing step in OGGM.

[7]

The area of OGGM’s flowline glaciers is corrected to the one provided by the RGI, for area conservation and inter-comparison reasons. If you do not want to use the RGI area but the one computed from the shape geometry in the local OGGM map projection instead, set cfg.PARAMS['use_rgi_area'] to False. This is useful when using homemade inventories.

Topography data

When creating a Glacier directories, a suitable topographical data source is chosen automatically, depending on the glacier’s location. OGGM supports a large number of datasets (almost all of the freely available ones, we hope). They are listed on the RGI-TOPO website.

The current default is to use COPDEM90 and COPDEM30. In practice, COPDEM is sufficient for all but about 300 of the world’s glaciers. The we use some other datasets available in RGITOPO.

These data are downloaded only when needed (i.e. during an OGGM run) and they are stored in the dl_cache_dir directory. The gridded topography is then reprojected and resampled to the local glacier map. The local grid is defined on a Transverse Mercator projection centered over the glacier, and has a spatial resolution depending on the glacier size. The default in OGGM is to use the following rule:

\[\Delta x = d_1 \sqrt{S} + d_2\]

where \(\Delta x\) is the grid spatial resolution (in m), \(S\) the glacier area (in km\(^{2}\)) and \(d_1\), \(d_2\) some parameters (set to 14 and 10, respectively). If the chosen spatial resolution is larger than 200 m (\(S \ge\) 185 km\(^{2}\)) we clip it to this value.

Important: when using these data sources for your OGGM runs, please refer to the original data provider of the data! OGGM adds a dem_source.txt file in each glacier directory specifying how to cite these data. We reproduce this information here.

Warning

A number of glaciers will still suffer from poor topographic information. Either the errors are large or obvious (in which case the model won’t run), or they are left unnoticed. The importance of reliable topographic data for global glacier modelling cannot be emphasized enough.

Note

In this blogpost we talk about which requirements a DEM must fulfill to be helpful to OGGM. And we also explain why and how we preprocess some DEMs before we make them available to the OGGM workflow.