[R-sig-Geo] How set up spatstat::densityHeat()?

[Michal Kvasnička]

Hello.

I have to find hotspots of road accidents. It seems that
spatstat::densityHeat() implements McSwiggan et al. (2017) algorithm I want
to use. I plan to follow path of DrHotNet package:

1. estimate the density function
2. lixelize the network
3. calculate the density in the middle point of each lixel from (2) using
function estimated in (1)
4. find clusters of lixels from (3) with sufficiently high density

However, I don't know how to set the function's parameters.

I cannot use the implicit setting (finespacing=TRUE) because it suggests
tens of billions iterations. (I use OSM road maps of Czech districts
converted to Krovak projection. My testing district has about 2,700 km of
roads with an average length of road segments of about 30 meters due to
road curvatures. This is not the largest district I have to handle---Prague
is much larger.)

I can set finespacing = FALSE and then finetune the parameters myself.
However, I can't find any documentation. Having skimmed the source code,
I'm trying something like this:

density_function <- densityHeat(accident_lpp, sigma, finespacing = FALSE,
eps = 10, fun = TRUE)

This way I get the function I need. However, I'm not certain what I'm
doing. It seems that the densityHeat() uses some grid, based on lixelized
network. My questions are:

* Is it sufficient to set the eps parameter or do I need to set other
parameters too? Which ones?
* Is the parameter in meters (my map is projected with meters as its units)?
* If eps = 10 means grid of 10x10 meters, what precisely does it mean? How
is the network lixelized (and why)? How does it affect the estimated
function (not the linim object)?
* What is the (economically/substantially) reasonable value of eps (and
other necessary parameters)?
* Is there any documentation to the function? (I've read chapter 17 of the
spatstat book but found nothing there. The function help page doesn't cover
this either.)
* Does it any difference if I lixelize the network myself before running
densityHeat()?

Many thanks for your advice. (And an apology for such a long question.)

Best wishes,
*Ing. Michal Kvasnička, Ph.D.*



*Masaryk University | School of Economics and Administration*
Department of Economics
A: Lipová 41a | 602 00 Brno
E: michal.kvasnicka using econ.muni.cz | W: www.econ.muni.cz/~qasar

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