SDO_DISTANCE for SRID=4156

Question

Hello, I have problem with distance.
I created the elementary example:
select sdo_geom.sdo_distance(
SDO_GEOMETRY(2001,null,SDO_POINT_TYPE(1, 1, NULL),
NULL,NULL),
sdo_geometry(2001,null,SDO_POINT_TYPE(2, 1, NULL),
NULL,NULL),
0.5) as dist
FROM dual;
Result is 1.
I created same example, but for SRID 4156 as you can see.
SELECT SDO_GEOM.SDO_DISTANCE(
SDO_GEOMETRY(2001,4156,SDO_POINT_TYPE(1, 1, NULL),
NULL,NULL),
sdo_geometry(2001,4156,SDO_POINT_TYPE(2, 1, NULL),
NULL,NULL),
0.5,
'unit=M') as dist
FROM dual;
Result is 111289,738222679
How is possible that result is soo different?
The orientation of axis in the S-JTSK(for Czech Republic) with SRID 4156 are switched, but for this elementary example should be result still one.

Answer 1

Hi Radan,
Well, there are some larger issues here beyond measurement. Your first example has a NULL srid, which means this is "pure" cartesian planar coordinate system. You might think of it as an infinitely large sheet of graph paper (or eastern european notebook paper). Its perfectly flat and uniform and thus distance is just a matter of high school geometry reported in a generic unit - if you try to ask for meters or miles or whatever you will get ORA-13293. There is no mapping to the actual earth. Whether you measure the distance between 1 and 2 or 1,000,000 and 1,000,001, its all the same out in to infinity - just one generic unit.
Now 4156 is a geodetic projection using decimal degrees as its unit of measure intended for usage around the Czech Republic and Slovakia.
http://spatialreference.org/ref/epsg/4156/
Neither decimal degrees nor the same in meters is comparable to that generic distance reported above. Now another question is why you are measuring coordinates down in the Gulf of Guinea with a Czech SRID? Still, its works well enough. If we measure the same distance with WGS84 we get 111302.65 meters.
If you are looking for some kind of "default" SRID for testing or comparison, I guess I would say use WGS84 (4326).
Cheers,
Paul