I have some experience with R as a statistics platform, but am inexperienced in image based maths. I have a series of photographs (tiff format, px/µm is known) with holes and irregular curves. I'd like to measure the shortest distance between a hole and the closest curve for that particular hole. I'd like to do this for each hole in a photograph. The holes are not regular either, so maybe I'd need to tell the program what are holes and what are curves (ImageJ has a point and segmented line functions).
Any ideas how to do this? Which package should I use in R? Would you recommend another program for this kind of task?
EDIT: Doing this is now possible using sclero package. The package is currently available on GitHub and the procedure is described in detail in the tutorial. Just to illustrate, I use an example from the tutorial:
library(devtools)
install_github("MikkoVihtakari/sclero", dependencies = TRUE)
library(sclero)
path <- file.path(system.file("extdata", package = "sclero"), "shellspots.zip")
dat <- read.ijdata(path, scale = 0.7812, unit = "um")
shell <- convert.ijdata(dat)
aligned <- spot.dist(shell)
plot(aligned)
It is also possible to add sample spot sizes using the functions provided by the sclero package. Please see Section 2.5 in the tutorial.
There's a tool for edge detection written for Image J that might help you first find the holes and the lines, and clarify them. You find it at
http://imagejdocu.tudor.lu/doku.php?id=plugin:filter:edge_detection:start
Playing around with the settings for the tresholding and the hysteresis can help in order to get the lines and holes found. It's difficult to tell whether this has much chance of working without seeing your actual photographs, but a colleague of mine had good results using this tool on FRAP images. I programmed a ImageJ tool that can calculate recoveries in FRAP analysis based on those images. You might get some ideas for yourself when looking at the code (see: http://imagejdocu.tudor.lu/doku.php?id=plugin:analysis:frap_normalization:start )
The only way I know you can work with images, is by using EBImage that's contained in the bioconductor system. The package Rimage is orphaned, so is no longer maintained.
To find the shortest distance: once you have the coordinates of the lines and holes, you can go for the shotgun approach : calculate the distances between all points and the line, and then take the minimum. An illustration about that in R :
x <- -100:100
x2 <- seq(-70,-50,length.out=length(x)/4)
a.line <- list(x = x,
y = 4*x + 5)
a.hole <- list(
x = c(x2,rev(x2)),
y = c(200 + sqrt(100-(x2+60)^2),
rev(200 - sqrt(100-(x2+60)^2)))
)
plot(a.line,type='l')
lines(a.hole,col='red')
calc.distance <- function(line,hole){
mline <- matrix(unlist(line),ncol=2)
mhole <- matrix(unlist(hole),ncol=2)
id1 <- rep(1:nrow(mline),nrow(mhole))
id2 <- rep(1:nrow(mhole), each=nrow(mline))
min(
sqrt(
(mline[id1,1]-mhole[id2,1])^2 +
(mline[id1,2]-mhole[id2,2])^2
)
)
}
Then :
> calc.distance(a.line,a.hole)
[1] 95.51649
Which you can check mathematically by deriving the equations from the circle and the line. This goes fast enough if you don't have millions of points describing thousands of lines and holes.
Related
first post :)
I've been transitioning my R code from sp() to sf()/stars(), and one thing I'm still trying to grasp is accounting for the area in my grids.
Here's an example code to explain what I mean.
library(stars)
library(tidyverse)
# Reading in an example tif file, from stars() vignette
tif = system.file("tif/L7_ETMs.tif", package = "stars")
x = read_stars(tif)
x
# Get areas for each grid of the x object. Returns stars object with "area" in units of [m^2]
x_area <- st_area(x)
x_area
I tried loosely adopting code from this vignette (https://github.com/r-spatial/stars/blob/master/vignettes/stars5.Rmd) to divide each value in x by it's grid area, and it's not working as expected (perhaps because my objects are stars and not sf?)
x$test1 = x$L7_ETMs.tif / x_area # Some computationally intensive calculation seems to happen, but doesn't produce the results I expect?
x$test1 = x$L7_ETMs.tif / x_area$area # Throws error, "non-conformable arrays"
What does seem to work is the following.
x %>%
mutate(test1 = L7_ETMs.tif / units::set_units(as.numeric(x_area$area), m^2))
Here are the concerns I have with this code.
I worry that as I turn the x_area$area (a matrix, areas in lat/lon) into a numeric vector, I may mess up the lat/lon matching between the grid and it's area. I did some rough testing to see if the areas match up the way I expect them to, but can't escape the worry that this could lead to errors that are difficult to catch.
It just doesn't seem clean that I start with "x_area" in the correct units, only to remove then set the units again during the computation.
Can someone suggest a "cleaner" implementation for what I'm trying to do, i.e. multiplying or dividing grids by its area while maintaining units throughout? Or convince me that the code I have is fine?
Thanks!
I do not know how to improve the stars code, but you can compare the results you get with this
tif <- system.file("tif/L7_ETMs.tif", package = "stars")
library(terra)
r <- rast(tif)
a <- cellSize(r, sum=FALSE)
x <- r / a
With planar data you could do this when it is safe to assume there is no distortion (generally not the case, but it can be the case)
y <- r / prod(res(r))
I found some posts and discussions about the above, but I'm not sure... could someone please check if I am doing anything wrong?
I have a set of N points of the form (x,y,z). The x and y coordinates are independent variables that I choose, and z is the output of a rather complicated (and of course non-analytical) function that uses x and y as input.
My aim is to find a set of values of (x,y) where z=z0.
I looked up this kind of problem in R-related forums, and it appears that I need to interpolate the points first, perhaps using a package like akima or fields.
However, it is less clear to me: 1) if that is necessary, or the basic R functions that do the same are sufficiently good; 2) how I should use the interpolated surface to generate a correct matrix of the desired (x,y,z=z0) points.
E.g. this post seems somewhat related to the problem I am describing, but it looks extremely complicated to me, so I am wondering whether my simpler approach is correct.
Please see below some example code (not the original one, as I said the generating function for z is very complicated).
I would appreciate if you could please comment / let me know if this approach is correct / suggest a better one if applicable.
df <- merge(data.frame(x=seq(0,50,by=5)),data.frame(y=seq(0,12,by=1)),all=TRUE)
df["z"] <- (df$y)*(df$x)^2
ta <- xtabs(z~x+y,df)
contour(ta,nlevels=20)
contour(ta,levels=c(1000))
#why are the x and y axes [0,1] instead of showing the original values?
#and how accurate is the algorithm that draws the contour?
li2 <- as.data.frame(contourLines(ta,levels=c(1000)))
#this extracts the contour data, but all (x,y) values are wrong
require(akima)
s <- interp(df$x,df$y,df$z)
contour(s,levels=c(1000))
li <- as.data.frame(contourLines(s,levels=c(1000)))
#at least now the axis values are in the right range; but are they correct?
require(fields)
image.plot(s)
fancier, but same problem - are the values correct? better than the akima ones?
what are some good kriging/interpolation idea/options that will allow heavily-weighted points to bleed over lightly-weighted points on a plotted R map?
the state of connecticut has eight counties. i found the centroid and want to plot poverty rates of each of these eight counties. three of the counties are very populated (about 1 million people) and the other five counties are sparsely populated (about 100,000 people). since the three densely-populated counties have more than 90% of the total state population, i would like those the three densely-populated counties to completely "overwhelm" the map and impact other points across the county borders.
the Krig function in the R fields package has a lot of parameters and also covariance functions that can be called, but i'm not sure where to start?
here is reproducible code to quickly produce a hard-bordered map and then three differently-weighted maps. hopefully i can just make changes to this code, but perhaps it requires something more complex like the geoRglm package? two of the three weighted maps look almost identical, despite one being 10x as weighted as the other..
https://raw.githubusercontent.com/davidbrae/swmap/master/20141001%20how%20to%20modify%20the%20Krig%20function%20so%20a%20huge%20weight%20overwhelms%20nearby%20points.R
thanks!!
edit: here's a picture example of the behavior i want-
disclaimer - I am not an expert on Krigging. Krigging is complex and takes a good understanding of the underlying data, the method and the purpose to achieve the correct result. You may wish to try to get input from #whuber [on the GIS Stack Exchange or contact him through his website (http://www.quantdec.com/quals/quals.htm)] or another expert you know.
That said, if you just want to achieve the visual effect you requested and are not using this for some sort of statistical analysis, I think there are some relatively simple solutions.
EDIT:
As you commented, though the suggestions below to use theta and smoothness arguments do even out the prediction surface, they apply equally to all measurements and thus do not extend the "sphere of influence" of more densely populated counties relative to less-densely populated. After further consideration, I think there are two ways to achieve this: by altering the covariance function to depend on population density or by using weights, as you have. Your weighting approach, as I wrote below, alters the error term of the krigging function. That is, it inversely scales the nugget variance.
As you can see in the semivariogram image, the nugget is essentially the y-intercept, or the error between measurements at the same location. Weights affect the nugget variance (sigma2) as sigma2/weight. Thus, greater weights mean less error at small-scale distances. This does not, however, change the shape of the semivariance function or have much effect on the range or sill.
I think that the best solution would be to have your covariance function depend on population. however, I'm not sure how to accomplish that and I don't see any arguments to Krig to do so. I tried playing with defining my own covariance function as in the Krig example, but only got errors.
Sorry I couldn't help more!
Another great resource to help understand Krigging is: http://www.epa.gov/airtrends/specialstudies/dsisurfaces.pdf
As I said in my comment, the sill and nugget values as well as the range of the semivariogram are things you can alter to affect the smoothing. By specifying weights in the call to Krig, you are altering the variance of the measurement errors. That is, in a normal use, weights are expected to be proportional to the accuracy of the measurement value so that higher weights represent more accurate measurements, essentially. This isn't actually true with your data, but it may be giving you the effect you desire.
To alter the way your data is interpolated, you can adjust two (and many more) parameters in the simple Krig call you are using: theta and smoothness. theta adjusts the semivariance range, meaning that measured points farther away contribute more to the estimates as you increase theta. Your data range is
range <- data.frame(lon=range(ct.data$lon),lat=range(ct.data$lat))
range[2,]-range[1,]
lon lat
2 1.383717 0.6300484
so, your measurement points vary by ~1.4 degrees lon and ~0.6 degrees lat. Thus, you can play with specifying your theta value in that range to see how that affects your result. In general, a larger theta leads to more smoothing since you are drawing from more values for each prediction.
Krig.output.wt <- Krig( cbind(ct.data$lon,ct.data$lat) , ct.data$county.poverty.rate ,
weights=c( size , 1 , 1 , 1 , 1 , size , size , 1 ),Covariance="Matern", theta=.8)
r <- interpolate(ras, Krig.output.wt)
r <- mask(r, ct.map)
plot(r, col=colRamp(100) ,axes=FALSE,legend=FALSE)
title(main="Theta = 0.8", outer = FALSE)
points(cbind(ct.data$lon,ct.data$lat))
text(ct.data$lon, ct.data$lat-0.05, ct.data$NAME, cex=0.5)
Gives:
Krig.output.wt <- Krig( cbind(ct.data$lon,ct.data$lat) , ct.data$county.poverty.rate ,
weights=c( size , 1 , 1 , 1 , 1 , size , size , 1 ),Covariance="Matern", theta=1.6)
r <- interpolate(ras, Krig.output.wt)
r <- mask(r, ct.map)
plot(r, col=colRamp(100) ,axes=FALSE,legend=FALSE)
title(main="Theta = 1.6", outer = FALSE)
points(cbind(ct.data$lon,ct.data$lat))
text(ct.data$lon, ct.data$lat-0.05, ct.data$NAME, cex=0.5)
Gives:
Adding the smoothness argument, will change the order of the function used to smooth your predictions. The default is 0.5 leading to a second-order polynomial.
Krig.output.wt <- Krig( cbind(ct.data$lon,ct.data$lat) , ct.data$county.poverty.rate ,
weights=c( size , 1 , 1 , 1 , 1 , size , size , 1 ),
Covariance="Matern", smoothness = 0.6)
r <- interpolate(ras, Krig.output.wt)
r <- mask(r, ct.map)
plot(r, col=colRamp(100) ,axes=FALSE,legend=FALSE)
title(main="Theta unspecified; Smoothness = 0.6", outer = FALSE)
points(cbind(ct.data$lon,ct.data$lat))
text(ct.data$lon, ct.data$lat-0.05, ct.data$NAME, cex=0.5)
Gives:
This should give you a start and some options, but you should look at the manual for fields. It is pretty well-written and explains the arguments well.
Also, if this is in any way quantitative, I would highly recommend talking to someone with significant spatial statistics know how!
Kriging is not what you want. (It is a statistical method for accurate--not distorted!--interpolation of data. It requires preliminary analysis of the data--of which you do not have anywhere near enough for this purpose--and cannot accomplish the desired map distortion.)
The example and the references to "bleed over" suggest considering an anamorph or area cartogram. This is a map which will expand and shrink the areas of the county polygons so that they reflect their relative population while retaining their shapes. The link (to the SE GIS site) explains and illustrates this idea. Although its answers are less than satisfying, a search of that site will reveal some effective solutions.
lot's of interesting comments and leads above.
I took a look at the Harvard dialect survey to get a sense for what you are trying to do first. I must say really cool maps. And before I start in on what I came up with...I've looked at your work on survey analysis before and have learned quite a few tricks. Thanks.
So my first take pretty quickly was that if you wanted to do spatial smoothing by way of kernel density estimation then you need to be thinking in terms of point process models. I'm sure there are other ways, but that's where I went.
So what I do below is grab a very generic US map and convert it into something I can use as a sampling window. Then I create random samples of points within that region, just pretend those are your centroids. After I attach random values to those points and plot it up.
I just wanted to test this conceptually, which is why I didn't go through the extra steps to grab cbsa's and also sorry for not projecting, but I think these are the fundamentals. Oh and the smoothing in the dialect study is being done over the whole country. I think. That is the author is not stratifying his smoothing procedure within polygons....so I just added states at the end.
code:
library(sp)
library(spatstat)
library(RColorBrewer)
library(maps)
library(maptools)
# grab us map from R maps package
usMap <- map("usa")
usIds <- usMap$names
# convert to spatial polygons so this can be used as a windo below
usMapPoly <- map2SpatialPolygons(usMap,IDs=usIds)
# just select us with no islands
usMapPoly <- usMapPoly[names(usMapPoly)=="main",]
# create a random sample of points on which to smooth over within the map
pts <- spsample(usMapPoly, n=250, type='random')
# just for a quick check of the map and sampling locations
plot(usMapPoly)
points(pts)
# create values associated with points, be sure to play aroud with
# these after you get the map it's fun
vals <-rnorm(250,100,25)
valWeights <- vals/sum(vals)
ptsCords <- data.frame(pts#coords)
# create window for the point pattern object (ppp) created below
usWindow <- as.owin(usMapPoly)
# create spatial point pattern object
usPPP <- ppp(ptsCords$x,ptsCords$y,marks=vals,window=usWindow)
# create colour ramp
col <- colorRampPalette(brewer.pal(9,"Reds"))(20)
# the plots, here is where the gausian kernal density estimation magic happens
# if you want a continuous legend on one of the sides get rid of ribbon=FALSE
# and be sure to play around with sigma
plot(Smooth(usPPP,sigma=3,weights=valWeights),col=col,main=NA,ribbon=FALSE)
map("state",add=TRUE,fill=FALSE)
example no weights:
example with my trivial weights
There is obviously a lot of work in between this and your goal of making this type of map reproducible at various levels of spatial aggregation and sample data, but good luck it seems like a cool project.
p.s. initially I did not use any weighting, but I suppose you could provide weights directly to the Smooth function. Two example maps above.
I'm trying to do the same thing asked in this question, Cartogram + choropleth map in R, but starting from a SpatialPolygonsDataFrame and hoping to end up with the same type of object.
I could save the object as a shapefile, use scapetoad, reopen it and convert back, but I'd rather have it all within R so that the procedure is fully reproducible, and so that I can code dozens of variations automatically.
I've forked the Rcartogram code on github and added my efforts so far here.
Essentially what this demo does is create a SpatialGrid over the map, look up the population density at each point of the grid and convert this to a density matrix in the format required for cartogram() to work on. So far so good.
But, how to interpolate the original map points based on the output of cartogram()?
There are two problems here. The first is to get the map and grid into the same units to allow interpolation. The second is to access every point of every polygon, interpolate it, and keep them all in right order.
The grid is in grid units and the map is in projected units (in the case of the example longlat). Either the grid must be projected into longlat, or the map into grid units. My thought is to make a fake CRS and use this along with the spTransform() function in package(rgdal), since this handles every point in the object with minimal fuss.
Accessing every point is difficult because they are several layers down into the SpPDF object: object>polygons>Polygons>lines>coords I think. Any ideas how to access these while keeping the structure of the overall map intact?
This problem can be solved with the getcartr package, available on Chris Brunsdon's GitHub, as beautifully explicated in this blog post.
The quick.carto function does exactly what you want -- takes a SpatialPolygonsDataFrame as input and has a SpatialPolygonsDataFrame as output.
Reproducing the essence of the example in the blog post here in case the link goes dead, with my own style mixed in & typos fixed:
(Shapefile; World Bank population data)
library(getcartr)
library(maptools)
library(data.table)
world <- readShapePoly("TM_WORLD_BORDERS-0.3.shp")
#I use data.table, see blog post if you want a base approach;
# data.table wonks may be struck by the following step as seeming odd;
# see here: http://stackoverflow.com/questions/32380338
# and here: https://github.com/Rdatatable/data.table/issues/1310
# for some background on what's going on.
world#data <- setDT(world#data)
world.pop <- fread("sp.pop.totl_Indicator_en_csv_v2.csv",
select = c("Country Code", "2013"),
col.names = c("ISO3", "pop"))
world#data[world.pop, Population := as.numeric(i.pop), on = "ISO3"]
#calling quick.carto has internal calls to the
# necessary functions from Rcartogram
world.carto <- quick.carto(world, world$Population, blur = 0)
#plotting with a color scale
x <- world#data[!is.na(Population), log10(Population)]
ramp <- colorRampPalette(c("navy", "deepskyblue"))(21L)
xseq <- seq(from = min(x), to = max(x), length.out = 21L)
#annoying to deal with NAs...
cols <- ramp[sapply(x, function(y)
if (length(z <- which.min(abs(xseq - y)))) z else NA)]
plot(world.carto, col = cols,
main = paste0("Cartogram of the World's",
" Population by Country (2013)"))
I need to make a topographic map of a terrain for which I have only fairly sparse samples of (x, y, altitude) data. Obviously I can't make a completely accurate map, but I would like one that is in some sense "smooth". I need to quantify "smoothness" (probably the reciprocal the average of the square of the surface curvature) and I want to minimize an objective function that is the sum of two quantities:
The roughness of the surface
The mean square distance between the altitude of the surface at the sample point and the actual measured altitude at that point
Since what I actually want is a topographic map, I am really looking for a way to construct contour lines of constant altitude, and there may be some clever geometric way to do that without ever having to talk about surfaces. Of course I want contour lines also to be smooth.
Any and all suggestions welcome. I'm hoping this is a well-known numerical problem. I am quite comfortable in C and have a working knowledge of FORTRAN. About Matlab and R I'm fairly clueless.
Regarding where our samples are located: we're planning on roughly even spacing, but we'll take more samples where the topography is more interesting. So for example we'll sample mountainous regions more densely than a plain. But we definitely have some choices about sampling, and could take even samples if that simplifies matters. The only issues are
We don't know how much terrain we'll need to map in order to find features that we are looking for.
Taking a sample is moderately expensive, on the order of 10 minutes. So sampling a 100x100 grid could take a long time.
Kriging interpolation may be of some use for smoothly interpolating your sparse samples.
R has many different relevant tools. In particular, have a look at the spatial view. A similar question was asked in R-Help before, so you may want to look at that.
Look at the contour functions. Here's some data:
x <- seq(-3,3)
y <- seq(-3,3)
z <- outer(x,y, function(x,y,...) x^2 + y^2 )
An initial plot is somewhat rough:
contour(x,y,z, lty=1)
Bill Dunlap suggested an improvement: "It often works better to fit a smooth surface to the data, evaluate that surface on a finer grid, and pass the result to contour. This ensures that contour lines don't cross one another and tends to avoid the spurious loops that you might get from smoothing the contour lines themselves. Thin plate splines (Tps from library("fields")) and loess (among others) can fit the surface."
library("fields")
contour(predict.surface(Tps(as.matrix(expand.grid(x=x,y=y)),as.vector(z))))
This results in a very smooth plot, because it uses Tps() to fit the data first, then calls contour. It ends up looking like this (you can also use filled.contour if you want it to be shaded):
For the plot, you can use either lattice (as in the above example) or the ggplot2 package. Use the geom_contour() function in that case. An example can be found here (ht Thierry):
ds <- matrix(rnorm(100), nrow = 10)
library(reshape)
molten <- melt(data = ds)
library(ggplot2)
ggplot(molten, aes(x = X1, y = X2, z = value)) + geom_contour()
Excellent review of contouring algorithm, you might need to mesh the surface first to interpolate onto a grid.
maybe you can use:
GEOMap
geomapdata
gtm
with
Matrix
SparseM
slam
in R