我想使用以下代码进行研究。但是每当我运行包含over（）和proj4string（）等的代码时，就会出现以下错误“错误（函数（类，fdef，mtable）： 找不到签名“” data.frame”，“ character”的函数“ proj4string <-”的继承方法。那么是否有任何选择编写此代码来避免这些功能。我的PC或R程序似乎出了点问题。
require(sp)
require(rgdal)
require(maps)
# read in bear data, and turn it into a SpatialPointsDataFrame
bears <- read.csv("bear-sightings.csv")
coordinates(bears) <- c("longitude", "latitude")
# read in National Parks polygons
parks <- readOGR(".", "10m_us_parks_area")
# tell R that bear coordinates are in the same lat/lon reference system
# as the parks data -- BUT ONLY BECAUSE WE KNOW THIS IS THE CASE!
proj4string(bears) <- proj4string(parks)
# combine is.na() with over() to do the containment test; note that we
# need to "demote" parks to a SpatialPolygons object first
inside.park <- !is.na(over(bears, as(parks, "SpatialPolygons")))
# use 'over' again, this time with parks as a SpatialPolygonsDataFrame
# object, to determine which park (if any) contains each sighting, and
# store the park name as an attribute of the bears data
bears$park <- over(bears, parks)$Unit_Name
# draw a map big enough to encompass all points (but don't actually plot
# the points yet), then add in park boundaries superimposed upon a map
# of the United States
plot(coordinates(bears), type="n")
map("world", region="Spain", add=TRUE)
plot(parks, border="green", add=TRUE)
legend("topright", cex=0.85,
c("Bear in park", "Bear not in park", "Park boundary"),
pch=c(16, 1, NA), lty=c(NA, NA, 1),
col=c("red", "grey", "green"), bty="n")
title(expression(paste(italic("Ursus arctos"),
" sightings with respect to national parks")))
我的目的是删除公园外的地点。之后，根据公园内点的位置选择NDVI图像的值。