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Intro to SAP HANA Geospatial - Spatial Reference Systems

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Intro to SAP HANA Geospatial - Spatial Reference Systems

By Vitaliy Rudnytskiy

In the context of spatial databases, the defined space in which geometries are described is called a spatial reference system (SRS).

Details

You will learn

You will learn what Spatial Reference Systems and Spatial Units of Measure are, how they are defined and used in SAP HANA.


Step 1: Calculating the distance on the flat surface

What is the distance between two points: (0, 0) and (1, 0)?

select new st_point('POINT (0 0)').st_distance(new st_point('POINT (1 0)')) as distance from dummy;
Distance in SRS=0

The result is 1.

1, but in what units? In this example the default unit of distance is meter, and conversion to other units is possible in some spatial methods. The list of pre-installed units is available in the system view ST_UNITS_OF_MEASURE. One of them is yard, used in the following example.

select new st_point('POINT (0 0)').st_distance(new st_point('POINT (1 0)'), 'yard') as distance from dummy;
Convert to yard

More units of measures can be manually created or loaded into the SAP HANA using separate Geospatial Metadata Installer. Geospatial Metadata Installer is available for registered customers and partners.

Step 2: Introducing Spatial Reference Systems

But what if you want to measure a distance between two geographic point where coordinates are latitude and longitude? Then you need to specify what Spatial Reference System (SRS) is used for geometries in queries.

A spatial reference system defines, at minimum:
- units of measure of the underlying coordinate system (degrees, meters, and so on),
- maximum and minimum coordinates (also referred to as the bounds),
- whether the data is planar or spheroid,
- projection information for transforming the data to other reference systems.

When not SRS is specified in the query, then the default SRS with id (SRID) equal 0 is assumed by SAP HANA. This is the one that was used in previous examples.

Spatial reference system details can be viewed in the ST_SPATIAL_REFERENCE_SYSTEMS system view.

Check some of the characteristics of SRS with SRID equal 0

select srs_name, round_earth, axis_order, min_x, max_x, linear_unit_of_measure
from ST_SPATIAL_REFERENCE_SYSTEMS where srs_id=0;
SRS 0 details

As you can see it is the DEFAULT reference system, with all x, y, z and m coordinates. It is based on the flat surface.

Step 3: Calculating the distance on the round Earth

The Spatial Reference System that implements geodetic coordinate system of the Global Positioning System (GPS), or WGS 84, has SRID 4326.

select srs_name, round_earth, axis_order, min_x, max_x, min_y, max_y, linear_unit_of_measure, angular_unit_of_measure
from ST_SPATIAL_REFERENCE_SYSTEMS where srs_id=4326;
SRS 4326

This SRS has degree defined as the angular unit of measure and boundaries are between -180 to 180 degrees longitude and -90 to 90 degrees latitude.

To calculate a distance of one degree of longitude at the equator in kilometers run this query.

select new st_point('POINT (0 0)', 4326).st_distance(new st_point('POINT (1 0)', 4326), 'kilometer') as distance from dummy;
one degree on the equator

A distance of one degree of longitude at the 45 degrees latitude should be shorter.

select new st_point('POINT (0 45)', 4326).st_distance(new st_point('POINT (1 45)', 4326), 'kilometer') as distance from dummy;
one degree on the 45deg latitude

SRS with id 4326 is the one of many standardized reference systems. Other may have different assumptions regarding the Earth shape, or may define boundaries to cover only selected countries.

Step 4: Defining additional Spatial Reference Systems

More spatial reference systems can be manually created or loaded into the SAP HANA using separate Geospatial Metadata Installer. Geospatial Metadata Installer is available for registered customers and partners.

For now create another popular reference system with SRID equal 3857. It is famous for being used in web maps, and you will use it later in another tutorials and exercises too.

CREATE SPATIAL REFERENCE SYSTEM "WGS 84 / Pseudo-Mercator"
 IDENTIFIED BY 3857
 DEFINITION 'PROJCS["WGS 84 / Pseudo-Mercator",GEOGCS["WGS 84",DATUM["WGS_1984",SPHEROID["WGS 84",6378137,298.257223563,AUTHORITY["EPSG","7030"]],AUTHORITY["EPSG","6326"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.0174532925199433,AUTHORITY["EPSG","9122"]],AUTHORITY["EPSG","4326"]],PROJECTION["Mercator_1SP"],PARAMETER["central_meridian",0],PARAMETER["scale_factor",1],PARAMETER["false_easting",0],PARAMETER["false_northing",0],UNIT["metre",1,AUTHORITY["EPSG","9001"]],AXIS["X",EAST],AXIS["Y",NORTH],EXTENSION["PROJ4","+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +wktext  +no_defs"],AUTHORITY["EPSG","3857"]]'
 ORGANIZATION "EPSG"
 IDENTIFIED BY 3857
 TRANSFORM DEFINITION '+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +wktext  +no_defs'   
 TYPE PLANAR
 COORDINATE X BETWEEN -20037508.342789248 AND 20037508.342789248 COORDINATE Y BETWEEN -20048966.104014635 AND 20048966.104014624  
 TOLERANCE DEFAULT
 SNAP TO GRID DEFAULT
 POLYGON FORMAT 'EvenOdd'
 STORAGE FORMAT 'Internal';

Confused about the whole cryptic notation in the definition? No need to worry for now. All you need to know is that its definition comes from industry registry EPSG Geodetic Parameter Registry that you can search and download if needed.

Once SQL above successfully executed you should see this new SRS in ST_SPATIAL_REFERENCE_SYSTEMS view.

Optional

Next Steps

Updated 01/12/2018

Time to Complete

15 Min

Beginner

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