Today I made maps.

Geophysics is a visual science. Nearly every paper I’ve written has started with the figures. The reason for this is that the end result of data analysis or calculation ends up making sense only as an image; the figures in my papers usually contain the results that the text seeks to obtain or interpret.

This dependence on imagery requires, like it or not, that I do some thinking about visual design and data presentation. The same information can often be presented several ways. Right now I’m putting together some results (collected by several people that I’ll credit properly in a future post) obtained using a measurement technique called shear-wave splitting. Skipping over what the measurement is, exactly (again for a later post), the ultimate result for a given seismometer station is two numbers: a direction (relative to north), and a magnitude (given in seconds). The map above uses the orthodox way of presenting this kind of information: double-headed[*] arrows oriented to follow the measured direction, with lengths scaled by the magnitude. This approach makes it easy to see how the splitting orientation relates to other features (such as the geological boundaries indicated by blue lines), but the arrows tend to overlap and obscure some of the spatial characteristics of the data (such as whether two points on opposite sides of the map have the same value).

Another way to do it is to separate the two:

Is this better, or worse? It does a better job of indicating that there’s a region in the centre of the map with high split times, which reinforces the point of the paper — at the same time, the azimuth is reduced to an abstraction, rather than drawing an angle as an angle.

In the paper I’m writing (actually an extended abstract for GeoCanada 2010), I think I’ll be using both — contours to show the extent, arrows to show the geological relationship. But maybe there’s a better graphical approach out there that I haven’t found yet.

As a postscript: I made these maps with a set of software tools called the Generic Map Tools, or GMT. They’re slightly obscure at times, but I find them better for my work than any GIS system I’ve fiddled with.

[*] Double-headed because the angle is out of 180 degrees rather than 360 — the method doesn’t distinguish between north-northwest and south-southeast.


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