[R-sig-Geo] Prevent minor discrepancy in pixel resolution using Spatstat

[Tyler Dean Rudolph]

When I specify eps=25 (m resolution) in a call to distmap...

> library(spatstat)
> temp <- data.frame(x=-670049.2, y=752814.6,  X2=-669961.9,  Y2=752648.2)
>        subwin<-owin(xrange=c(-100, 100) + sort(c(temp$x, temp$X2)),
yrange=c(-100, 100) + sort(c(temp$y, temp$Y2)))
>        trajpsp<-psp(temp$x,temp$y, temp$X2, temp$Y2, subwin, check=TRUE)
>        plot(trajmap<-distmap(trajpsp, eps=25))
> trajmap$xstep
[1] 23.94167
> trajmap$ystep
[1] 24.42667

...The resulting resolution is slightly different than 25x25 metres.  So now
when I use it as a mask:
>        trajmap$v<-ifelse(trajmap$v>50, 0, 1)
> library(maptools)

...and convert it to a RasterLayer for matrix algebra with another raster
> trajrast<-raster(as.SpatialGridDataFrame.im(from=trajmap))
> tempmap<-crop(habmap, extent(trajrast)) # tempmap.grd is attached
> top<-table(getValues(tempmap*trajrast))
Error in compare(c(e1, e2)) : Different bounding box
Error in getValues(tempmap * trajrast) :
  error in evaluating the argument 'x' in selecting a method for function
'getValues'

...it doesn't work because the resolution is not exactly equal.  Now I can
sometimes trick it when the number of rows and cells are still equal, BUT...
extent(trajrast)<-extent(tempmap)
> top<-table(getValues(tempmap*trajrast))
Error in compare(c(e1, e2)) : nrows different
Error in getValues(tempmap * trajrast) :
  error in evaluating the argument 'x' in selecting a method for function
'getValues'

...This is not always the case!

How can I prevent the numbers of rows and columns from being so slightly
different because of a minor different in resolution?  Otherwise put, how
can I make the actual resolution of my distmap image exactly 25m as desired?

Tyler
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