Srtm_relief remote data set and NaN

Q&A
1

Introduction

I made a map using the SRTM grid (which only contains land data) for the region covering the Dead Sea and the Mediterranean Sea. Since each sea has a different sea level, creating an accurate map can be challenging (as you can see in the examples below).

Question

The SRTM data in the Mediterranean Sea originally contains NaN values. However, as far as I know, GMT replaces these NaN values with 0 during processing.

Wouldn’t it be better to keep the original NaN values instead of replacing them with 0? Are there cases where replacing NaN with 0 is necessary or beneficial?

If such cases exist, perhaps GMT could offer an option to let users decide whether to replace NaN values with 0 or keep them unchanged.

Examples

gmt grdimage @srtm_relief_03s -Baf -R34:30/35:45/31:30/32 -png DeadSea -Cgeo+h-414

DeadSea
DeadSea1920×912 108 KB

gmt grdimage @srtm_relief_03s -Baf -R34:30/35:45/31:30/32 -png DeadSea

DeadSea
DeadSea1920×912 144 KB

2

SRTM data originally came in a Int16 type, so no NaNs there. I don’t think (though I can be wrong) that GMT is guilty of setting the ocean (water as a matter of fact) data to zero.

3

Ok, I have this impression because when I make a grid of the region of Israel (IL):
gmt grdcut @srtm_relief_03s -RIL -Ggrid.nc -V

I got this message:
grdblend [INFORMATION]: 7377360 nodes assigned values, 306240 set to 0

So, which nodes were assigned to 0? I thought it was the Sea/Ocean.