[R] Spatstat help - quadratcount query

[Gough Lauren]

Hi all,

I am using quadratcount in spatstat to divide a window containing a
point pattern into a grid of quadrats containing the intensity of points
in each quadrat.  My data is in UTM co-ordinates.  My window is defined
as follows:

>p15<-ppp(x,y,window=owin(c(341710,342100),c(3126465,3126780)),marks=NUL
L, checks=TRUE)

Giving me a distance of 390m in the 'x' direction and 315m in the 'y'
direction.  I want to divide the window into quadrats of size 15m x 15m
so I did the following:

>q15<-quadratcount(p15,nx=26,ny=21).

However, when I look at the data for quadrat counts it seems the
function is not keeping the size of the quadrats constant, but is
instead varying the width of the quadrats (in both the x and y
direction) between 10m and 20m, meaning that some quadrats are 10m x 10m
and some are four times the size (20m x 20m) (I have pasted some of the
output below to demonstrate).  This is quite concerning as I am trying
to ascertain the scale of variation in my point pattern density - I
cannot do this if the quadrat scale does not stay constant!!

Does anyone know how to make quadratcount produce equally sized
quadrats?

Many thanks

Lauren


	ymin	3126470	3126480	3126500	3126510	3126530
	ymax	3126480	3126500	3126510	3126530	3126540
xmin	xmax					
341710	341720	0	1	0	0	0
341720	341740	0	0	3	1	5
341740	341750	0	1	2	1	3
341750	341770	2	1	0	0	1
341770	341780	4	6	5	4	0
341780	341800	1	4	2	1	1
341800	341810	0	0	1	5	1
341810	341830	0	0	0	1	3
341830	341840	4	1	3	0	0
341840	341860	1	1	0	0	2
341860	341880	4	2	0	0	3
341880	341890	3	2	0	5	1
341890	341900	3	2	0	4	1
341900	341920	3	2	2	0	0
341920	341930	0	3	2	3	0
341930	341950	1	5	2	2	4
341950	341960	0	0	6	3	2
341960	341980	0	0	0	3	2
341980	341990	0	0	0	0	0
341990	342010	0	0	3	0	0
342010	342020	1	0	0	0	0
342020	342040	1	1	1	1	2
342040	342050	1	0	0	0	2
342050	342070	1	0	0	0	0
342070	342080	1	0	0	0	0
342080	342100	0	0	0	0	0

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