r elevatr AWS and USGS elevation are completely different
I am using the elevatr package in R and realized that some of elevation values I get when calling get_aws_points() are completely different than when calling get_elev_point(). In some cases they are thousands of feet apart. I have have checked the docs from AWS here: https://github.com/tilezen/joerd/blob/master/docs/data-sources.md and see that California should be using USGS 3DEP (either 3 or 10 meter resolution in California). The USGS point query service (https://nationalmap.gov/epqs/) uses a 1/3 arc-second dem (so 10m). These should essentially be the same dataset. I have creatted the following reproducible example here. Does anyone have a clue what is going on here? None of the returned elevations are identical. Both functions call elevation in meters and the z argument in get_aws_points() seems to have no effect on the returned elevation value.
library(tidyverse)
library(elevatr)
library(USAboundaries)
library(sf)
#> Linking to GEOS 3.9.3, GDAL 3.5.2, PROJ 8.2.1; sf_use_s2() is TRUE
# Compare get_aws_points and get_elev_points
# Import California Polygon and project to California Albers
ca.poly <- us_states()%>%
filter(state_name == "California")%>%
st_transform(3310)
# Create Sample Points
pts <- ca.poly%>%
st_make_grid(10000)%>%
st_centroid()%>%
st_sf()%>%
filter(st_intersects(., ca.poly, sparse = FALSE))
# Get elevation from AWS
e1 <- as.data.frame(get_aws_points(pts), z = 1)
#> Warning: PROJ support is provided by the sf and terra packages among others
#> Warning: PROJ support is provided by the sf and terra packages among others
#> Mosaicing & Projecting
#> Note: Elevation units are in meters.
pts$AWS_Meters <- e1$elevation
# Get Elevation with higher zoom from AWS
e2 <- as.data.frame(get_aws_points(pts), z = 14)
#> Warning: PROJ support is provided by the sf and terra packages among others
#> Warning: PROJ support is provided by the sf and terra packages among others
#> Mosaicing & Projecting
#> Note: Elevation units are in meters.
pts$AWS_Meters_Z14 <- e2$elevation
# Get elevation from USGS
e3 <- as.data.frame(get_elev_point(pts))
#> Warning: PROJ support is provided by the sf and terra packages among others
#> Downloading point elevations:
#> Note: Elevation units are in meters
pts$USGS_Meters <- e3$elevation
# Calculate difference
pts$elevDif <- pts$AWS_Meters - pts$USGS_Meters
summary(pts$elevDif)
#> Min. 1st Qu. Median Mean 3rd Qu. Max.
#> -599.0500 -26.4975 0.6650 -0.2794 31.6025 486.2700
# Plot difference as map
ggplot(pts)+
geom_sf(aes(color = elevDif), size = 1)+
scale_colour_gradientn(colours=rainbow(6))+
labs(title = "AWS Elevation - USGS Elevation [meters]",
color = "Meters")
# Plot difference as points
ggplot(pts)+
geom_abline(slope = 1, intercept = 0, color = "red", size = 1)+
geom_point(aes(x = AWS_Meters, y = USGS_Meters), alpha = 0.2)
#> Warning: Using `size` aesthetic for lines was deprecated in ggplot2 3.4.0.
#> ℹ Please use `linewidth` instead.
Created on 2022-11-22 with reprex v2.0.2
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Solution
Thanks for the question and detailed reprex.
The problem is in the two lines:
e1 <- as.data.frame(get_aws_points(pts), z = 1)
e2 <- as.data.frame(get_aws_points(pts), z = 14)
The z argument is an argument for get_aws_points. You are passing it as an argument for as.data.frame. As such, your elevations on e1 and e2 are both using the default z of 5 which returns a large pixel (several km). I wouldn't expect the USGS and the AWS with z of 5 elevations to very closely match.
If you change those two lines to:
e1 <- as.data.frame(get_aws_points(pts, z = 1))
e2 <- as.data.frame(get_aws_points(pts, z = 14))
Then you will get what you want. The e1 elevations will be very different than USGS, but the e2 elevations should be very close. Big caveat here... z=14 will return a raster for approximately all of California with an approximate pixel size of 7 meters. This will be large! I would scale this back. I tested it with a z = 10 and the difference between the two sources ranged from -81 to 52 meters. This is expected as the terrain tiles scale based on the zoom so the elevation at a given zoom level will be resampled. The USGS point service is pulling directly from a the 10m NED and is not resampled.
Also, since your point elevations are all in the US, I would use the USGS point elevation service for these data and not mess with the AWS source. The AWS point elevations are, admittedly, a bit of a hack to have a point elevations for global applications. There is no global analogue to the USGS service that I know of.
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