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I have a table of all Stop and Frisks that happened in 2017. I am given coordinates of where they happened in Long Island coordinates, but I want to convert it to latitude and longitude coordinates so that I can plot it in Leaflet.

I have the following snippet of code:

library(sp)
library(dplyr)

fd <- "https://www1.nyc.gov/assets/nypd/downloads/excel/analysis_and_planning/stop-question-frisk/sqf-2017.csv"
stop_and_frisk <- read.csv(fd)

saf <- stop_and_frisk %>% filter(STOP_FRISK_ID < 5 ) # filtering to keep data small
saf_spdf <- saf

coordinates(saf_spdf) <- ~STOP_LOCATION_X + STOP_LOCATION_Y
CRS_obj <- CRS('+proj=lcc +lat_1=41.03333333333333 +lat_2=40.66666666666666 +lat_0=40.16666666666666 +lon_0=-74 +x_0=300000.0000000001 +y_0=0 +ellps=GRS80 +datum=NAD83 +to_meter=0.3048006096012192 +no_defs')
spTransform(saf_spdf, CRS_obj)

I would expect that the coordinates would transform but instead I keep getting the error message

No transformation possible from NA reference system

And I'm not sure why. I haven't done very many CRS transformations before. I think the code above should sufficiently recreate the problem

Cameron
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1 Answers1

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The proj4string(same as CRS) isn't set, which explains the error: spTransform(): “No transformation possible from NA reference system”.

If you inspect the proj4string, you'll see that it is NA.

coordinates(saf_spdf) <- ~STOP_LOCATION_X + STOP_LOCATION_Y
proj4string(saf_spdf)

Retuns:

[1] NA

You need to first SET the proj4string of this object, and then you can transform it to the lat/lon that leaflet() takes.

# make data.frame a spatial object
coordinates(saf_spdf) <- ~STOP_LOCATION_X + STOP_LOCATION_Y

# SET the CRS of the object
proj4string(saf_spdf) <- CRS('the CRS of these coordinates as a character string')
# NOW we can transform to lat/lon
new <- spTransform(saf_spdf, CRS("+proj=longlat +ellps=WGS84 +datum=WGS84"))

# and finally, leaflet will accept this spatial object
new %>% leaflet()

Some intuition behind this error:

Spatial transforms work like this: (1) R needs to know what reference system you're in to begin with; (2) once we know what the starting reference system is, then and only then can we transform those points into a new reference system. You're getting this error because you haven't specified WHERE to begin. You can't transform something if you don't know where it's starting from. This like asking a computer to "multiply by 5". We need something to "multiply" in the first place! Setting a CRS tells R where to start the transformation.

Rich Pauloo
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  • I will give that a try! – Cameron Jul 12 '18 at 00:25
  • No luck unforunately. I'm still getting the same error message about NA reference system – Cameron Jul 12 '18 at 00:27
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    I took another look and just updated my answer. This should do the trick. – Rich Pauloo Jul 12 '18 at 00:28
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    Spatial transforms work like this: (1) R needs to know what reference system you're in to begin with; (2) once we know what the starting reference system is, then and only then can we **transform** those points into a **new** reference system. You're getting this error because you haven't specified **WHERE to begin**. You can't transform something if you don't know where it's starting from. This like asking a computer to "multiply by 5". We need something to "multiply" in the first place! Setting a CRS tells R where to start the transformation. – Rich Pauloo Jul 12 '18 at 00:32