# FAQ and Troubleshooting¶

We list here some of the questions we get most often, either on the OGGM Users mailing list or on github.

## General questions¶

### What is the difference between OGGM and other glacier models?¶

There are plenty of established ice dynamics models, and some of them are open-source (e.g. PISM, Elmer/Ice).

The purpose of OGGM is to be an easy to use, fully automated global glacier model, i.e. applicable to any glacier in the world without specific tuning or tweaking. Therefore, it does not attempt to replace (and even less compete with) these established ice dynamics models: it can be seen as a “framework”, a set of unified tools with eases the process of working with many mountain glaciers at once.

There is a standard modelling chain in OGGM (with a mass-balance model and a multiple flowline model) but there is no obligation to use all of these tools. For example, we can easily picture a workflow where people will use OGGM to create homogenized initial conditions (topography, climate) but use a higher order dynamical model like PISM instead of the simplified OGGM dynamics. For these kind of workflows, we created the OGGM-contrib example package which should help OGGM users to implement their own physics in OGGM.

### Can I use OGGM to simulate <my favourite glacier>?¶

The short answer is: “yes, but…”

The longer answer is that OGGM has been designed to work with all the world’s glaciers, and calibrated only on a few hundreds of them (and that’s only the mass-balance model…). We are quite confident that OGGM provides reasonable global estimates of glacier mass-balance and glacier change: this is a result of the law of large numbers, assuming that the uncertainty for each single glacier can be large but random and Gaussian.

If you use OGGM for a single or and handful of glaciers, chances are that the outcome is disappointing. For these kind of applications, you’ll probably need to re-calibrate OGGM using local data, for example of mass-balance or observations of past glacier change.

### Can I use OGGM to simulate long term glacier evolution?¶

It depends what you mean by “long-term”: at centenial time scales, probably, yes. At millenial time scales, maybe. At glacial time scales, probably not. The major issue we have to face with OGGM is that it uses a “glacier-centric” approach: it can simulate the mountain glaciers and ice-caps we know from contemporary inventories, but it cannot simulate glaciers which existed before but have disappeared today.

Also, if glaciers grow into large ice complexes and ice caps, the flowline assumption becomes much less valid than for typical valley glaciers found today. For these situations, fully distributed models like PISM are more appropriate.

We are currently in the process of testing and tuning OGGM for post-LIA simulations in the Alps. Reach out if you would like to know more about our progress.

### I have a question about OGGM, can we talk about it per email/phone?¶

Thanks for your interest in OGGM! Usually, we prefer to keep usage questions on github issues so that everybody can learn from all questions and their answers. You can also join our Slack discussion channel if you want a more interactive forum. Keep in touch with us per email if you’d like to join!

## Usage¶

### Can I export OGGM centerlines to a shapefile?¶

Yes! There is a function to do exactly that: utils.write_centerlines_to_shape().

## Troubleshooting¶

### Some glaciers exit with errors. What should I do?¶

Many things can go wrong when simulating all the world glaciers with a single model. We’ve tried our best, but still many glaciers cannot be simulated automatically. Possible reasons include complex glacier geometries that cannot be simulated by flowlines, very cold climates which don’t allow melting to occur, or numerical instabilities during the simulation. Altogether, 4218 glaciers (3.6% of the total area worldwide) could not be modelled by OGGM in the standard global simulations. Some regions experience more errors than others (see the paper).

When you experience errors, you have to decide if they are due to an error in your code or a problem in OGGM itself. The number and type of errors might help you out to decide if you want to act and troubleshoot them (see below). Also, always keep in mind that the number of errors is less important than the glacier area they represent. Plenty or errors on small glaciers is not as bad as one large glacier missing.

Then, you have to carefully consider how to deal with missing glaciers. Most studies will upscale diagnostic quantities using power laws or interpolation: for example, use volume-area-scaling to compute the volume of glaciers that are missing after an OGGM run. Importantly, you have to always be aware that these quantities will be missing from the compiled run outputs, and should be accounted for in quantitative analyses.

### What does the “Glacier exceeds domain boundaries” error mean?¶

This happens when a glacier grows larger than the original map boundaries. We recommend to increase the glacier map in this case, by setting cfg.PARAMS[‘border’] to a larger value, e.g. 100 or 200. The larger this value, the larger the glacier can grow (the drawback is that simulations become slowier and hungrier in memory because the number of grid points increases as well). We do not recommend to go larger than 250, however: for these cases it is likely that something else is wrong in your workflow or OGGM itself.

### What does the “NaN in numerical solution” error mean?¶

This happens when the ice dynamics simulation is unstable. In OGGM we use an adaptive time stepping scheme (which should avoid these kind of situations), but we also implemented thresholds for small time steps: i.e. if a simulation requires very small time steps we still use a larger one to avoid extremely slow runs. These thresholds are “bad practice” but required for operational reasons: when this happens, it is likely that the simulations blow up with a numerical error. There is not much you can do here, unless maybe set your own thresholds for small time steps (at the cost of computation time).

### Can I use my own Glacier inventory and outlines in OGGM?¶

You will be able to include your own inventory and outlines in OGGM, as long as the format of your shapefile is the same as the RGI file (v5 and v6 are supported). The attribute table should match the RGI format with the same amount of columns and variable names. See Glacier outlines and intersects for more information about the list of glacier attributes needed by OGGM. If you decide to use your own inventory (e.g. maybe because it has a better glacier outline) we encourage you to contact the GLIMPS core team to let them know how your inventory improves the glacier digitalization compared to the current RGI version. If you want to see an example on how to give OGGM a different shapefile than RGI, have a look at our online tutorial!