Can we take the length of a linestring by calculating ST_Distance from its ST_StartPoint and ST_EndPoint?

Question

I took the values of my StartPoint and EndPoint by doing this procedure:

a)

SELECT COUNT( CASE WHEN ST_NumGeometries(geom) > 1 THEN 1 END ) AS multi, COUNT(geom) AS total FROM public.roads;

because AS_Text(geom) was giving me multi-linestrings.the multi was 0 so I updated the geometry type to linestring:

b)

ALTER TABLE public.roads ALTER COLUMN geom TYPE geometry(LineString, 4326) USING ST_GeometryN(geom, 1);

and finaly

c)

INSERT INTO myschema.nodes SELECT ST_StartPoint(ST_GeometryN(ST_Multi(geometry),1)), ST_EndPoint(ST_GeometryN(ST_Multi(geometry),1)) FROM public.roads

is that procedure right?

and after that I was looking the way to find the length of my roads. the only command that worked to me was the:

SELECT ST_Length(geom::geography)/1000 from public.roads;

but I thought that maybe I can find it by:

SELECT ST_DISTANCE(st_p::geography,en_p::geography)/1000,weight FROM myschema.nodes

the results are different and I'd like to understand the reason.

I have not been working on GIS for very long and I'm a little bit confused.

Answer 1

The results are different because they are different functions.

ST_Length will return the length of the road (possibly multilinestring)

ST_Distance will return the straight distance between two points.

Imagine a road shaped like the letter C... ST_Distance of the start and end points of C will be much shorter than ST_Length of C.