Accurate channel digitizing implies following actual flow path including either emerging meanders in recent data or oxbow lakes that used to be a channel in the past. Connecting such segments may give a false impression of tremendous channel migration. These cases can be detected by analyzing curvature changes.
We are likely dealing with formation or collapse of a meander when the largest curvature changes between past and recent data in about at least an order of magnitude and either curvature is large enough for a visible meander.
The following plot might give you an idea how to pinpoint such cases.
library(rgdal) library(ggplot2) library(grid) Original <- readOGR("..", "lower_mn_1938") Final <- readOGR("..", "lower_mn_2009") Years <- 71 spar <- 0.4 Step_pattern <- 'symmetricP05' Curvature_multiplier <- 1e5 Step <- 25 source('~/.qgis2/processing/rscripts/Channel migration.rsx') Min_curvature <- 1e-3 source('~/.qgis2/processing/rscripts/Meander enumeration.rsx') source('~/.qgis2/processing/rscripts/Meander summary.rsx') transp <- element_rect(fill='transparent') qplot(abs(c_from)+abs(c_to), log10(abs(c_from/c_to)), size=len, data=Meanders@data) + annotation_custom(grob=rectGrob(gp=gpar(fill='gray60',alpha=.2)), xmin=.005, ymin=-3, xmax=.05, ymax=-1) + annotation_custom(grob=rectGrob(gp=gpar(fill='gray60',alpha=.2)), xmin=.005, ymin=1, xmax=.05, ymax=3) + annotate('text', .02, 1.5, label='collapsing meanders', size=8) + annotate('text', .02, -1.5, label='emergin meanders', size=8) + theme_bw() %+replace% theme(plot.background=element_rect(fill='transparent', colour='transparent'), legend.background=transp, legend.key=transp, panel.background=transp)
Figure 1: Change in maximum curvature for meanders