# Gauges¶

With AMRClaw in two space dimensions and GeoClaw it is possible to specify gauge locations as points (x,y) where the values of all components of q should be output every time step during the computation over some time range (t1,t2).

Gauges are useful in several ways, e.g.:

1. To compare computational results to measurements from physical gauges such as a pressure gauge or tide gauge that record data as a function of time at a single point,
2. To better visualize how the solution behaves at a single point,
3. To better compare results obtained with different methods or grid resolutions. Comparing two-dimensional pcolor or contour plots can be difficult whereas comparing to curves that give the solution as a function of time often reveals more clearly differences in accuracy or nonphysical oscillations.

## Gauge parameters in setrun¶

Gauges are specified in setrun by adding lists of gauge data for each desired gauge to the ClawRunData object rundata.gaugedata.gauges. This is initialized as an empty list and new gauges can be specified by:

rundata.gaugedata.gauges.append([gaugeno, x, y, t1, t2])


with values

• gaugeno : integer

the number of this gauge

• x, y : floats

the location of this gauge

• t1, t2 : floats

the time interval over which gauge data should be output.

During the computation the value of all components of q at all gauge locations will be output to a single file fort.gauge in the output directory. Lines of this file have the form:

gaugeno  level  t  q[0]  q[1] ...  q[meqn-1]


where level is the AMR level used to determine the q values at this time. Internally the finest level available at each gauge is used, with bilinear interpolation to the gauge locations from the 4 nearest cell centers.

New in 5.4.0. The output that is in the gauge files can be controlled by a variety of parameters. These can be specified on a per gauge basis or set for all gauges specified. The output parameters are

• file_format : Specifies the file format of the gauge data. Currently “ascii” is the only value accepted.
• display_format : Specifies the format of the numbers written to the gauge file for each field. These are Fortran format strings defaulting to “e15.7”.
• q_out_fields : Specifies which fields of the q array to output. Specify as a list the indices that should be output. Defaults to “all”.
• aux_out_fields : Specifies which fields of the aux array to output. Specify as a list the indices that should be output. Defaults to “none”
• min_time_increment : Specify a minimum amount of time that should pass before recording the values at a gauge. This can be useful for decreasing the amount of output at a gauge location that is currently being time-stepped at small increments. The default is 0 which effectively turns off this constraint.

Setting these values can be done in multiple ways for convenience. The most direct way is via a dictionary with the keys as the gauge ids and the corresponding parameter as the value. For example, if we had 3 gauges with ids 3, 7, 13 we could specify that they all use the display format e26.16 by setting:

rundata.gaugedata.display_format = "e26.16"


or:

rundata.gaugedata.display_format = {3:"e26.16", 13:"e8.6"}


to set gauge 3’s display format to “e26.16”, leave gauge 7 set to the default and set 13’s to “e8.6”.

For the parameters q_out_fields and aux_out_fields one can also specify “all” to output all fields or “none” to specify none of them (equivalent to an empty list of indices). Both of these arrays use Python indexing, i.e. they start at 0.

Note: For GeoClaw, the sea-surface value $$\eta = h + B$$ (sum of water depth and topography) is also output as another column after the q fields. In the case of the multilayer code the eta for each surface follows the q fields for that layer.

New in 5.4.0:

• Gauge output is accumulated in a buffer internally and written out intermitently, instead of writing to disk every time step. (The parameter MAX_BUFFER in the amrclaw library routines gauges_module.f90 controls the size of this buffer.)
• The gauge output for the gauges is written to distinct files in the output directory, e.g. gauge00001.txt for gauge number 1. In previous versions of Clawpack all gauges were written to a single file fort.gauge. The new approach allows gauges to be written in parallel and also facilitates reading in a single gauge more quickly.
• Some header info appears in each of these files to describe the gauge output.
• When doing a restart (see Checkpointing and restarting), gauge output from the original run is no longer overwritten by the second run. Instead gauge output from the restart run will be appended to the end of each gaugeXXXXX.txt file.

## Plotting tools¶

Several Python plotting tools are available to plot the gauge data, so you do not have to parse the file fort.gauge yourself.

If you want to read in the data for a particular gauge to manipulate it yourself, you can do, for example:

from clawpack.pyclaw.gauges import GaugeSolution
g = GaugeSolution(gauge_id=1, path='_output')


to examine gauge number 1, for example.

Then:

• g.t is the array of times,
• g.q is the array of values recorded at the gauges (g.q[m,n] is the mth variable at time t[n])

Alternatively, you can use the getgauge method of a ClawPlotData object, e.g.:

from clawpack.visclaw.data import ClawPlotData
plotdata = ClawPlotData()
plotdata.outdir = '_output'   # set to the proper output directory
gaugeno = 1                   # gauge number to examine
g = plotdata.getgauge(gaugeno)


In the setplot Python script you can specify plots that are to be done for each gauge, similar to the manner in which you can specify plots that are to be done for each time frame. For example, to plot the component q[0] at each gauge, include in setplot lines of this form:

plotfigure = plotdata.new_plotfigure(name='q[0] at gauges', figno=300, \
type='each_gauge')

# Set up for axes in this figure:
plotaxes = plotfigure.new_plotaxes()
plotaxes.xlimits = 'auto'
plotaxes.ylimits = [-1.5, 1.5]
plotaxes.title = 'q[0]'

# Plot q[0] as blue line:
plotitem = plotaxes.new_plotitem(plot_type='1d_plot')
plotitem.plot_var = 0
plotitem.plotstyle = 'b-'


Note that plotdata.new_plotfigure is called with type=’each_gauge’ which denotes that this plot is to be produced for each gauge found in setgauges.data. (When type is not specified, the default is type=’each_frame’ for time frame data).

If you type: