NASA's Black Marble monthly data: Reprojection isn't accurate

[nikos_geography]

I downloaded NASA's Black Marble monthly nighttime light NTL data, VNP46A3. Ι am trying to set the projection to EPSG:4326 but the resulting product (All_Angles_Snow_Free) seems to go go way off. For example, I wanted to download NTL data for the city of Mumbai, India. After reprojecting the NTL (product 5 (All_Angles_Snow_Free) from the .h5) the result is shown in the attachement.

At the bottom if the image is a shp of Mumbai and the red circle in the top indicates where Mumbai is in the NTL image. Clearly something's not right.

I downloaded the image from here (LAADS-DAAC, Level-1 and Atmosphere Archive & Distribution System Distributed Active Archive Center). The code I used to extract the NTL radiance image is:

library(terra)

wd <- "path/"

r <- rast(paste0(wd, "VNP46A3.A2018091.h25v07.001.2021125122857.h5"))
crs(r) <- "epsg:4326"

2400*(15/(60*60))

h = 25
v = 7

ext(r) = c(-180+h*10,-180+(h+1)*10, (v-2)*10,(v-1)*10)

ntl <- r[[5]]
writeRaster(ntl, paste0(wd, "ntl.tif"), overwrite = TRUE)

What's wrong with the code? From here (https://drive.google.com/drive/folders/1V115zpdU2-5fXssI6iWv_F6aNu4E5qA7?usp=drive_link) you can download the .h5 image if you don't want to use NASA's website. I am using R 4.3.1 and RStudio 2023.06.2+561.

[LAADSx_UserServices_M]

Hi Nikolaos,
Yes, it appears that the issue lies in how you're calculating the spatial extent. This is how you calculate the bounding box using the horizontal (h) and vertical(v) time number –

WestBoundCoord = (10*HorizontalTileNumber) - 180
NorthBoundCoord = 90-(10*VerticalTileNumber)

EastBoundCoord = WestBoundCoord + 10
SouthBoundCoord = NorthBoundCoord - 10

I've noticed that you're using the R program to extract the VNP46A3 layer. We actually have a Python script that accomplishes the same task. Here is a script for you reference –
https://blackmarble.gsfc.nasa.gov/tools/OpenHDF5.py

[nikos_geography]

Thank you for the reply and I apologize for my late response. I found a very handy package in R that takes care of everything (reprojection, clipping to the desired extent, download daily, monthly and annual products etc). It called blackmarbler (https://github.com/worldbank/blackmarbler).

[LAADSx_UserServices_M]

Thanks very much for the information. Yes, we have this tool available from our black marble tools website at:
https://blackmarble.gsfc.nasa.gov/Tools.html