Can't calculate proximity between polygon & point

[kaxil]

I need to check whether a polygon is within 10 km of a point, but I am unable to do it.

I was able to calculate distance between 2 points using ST_GeodesicLengthWGS84 and ST_LineString.

Select ST_GeodesicLengthWGS84(ST_SetSRID(ST_LineString(array(ST_Point(33.30,42.0),ST_Point(3.0,4.0))), 4326))/1000 as
 dist, ts
from table1

But it seems impossible for me to do the same for calculating distance between Polygon and Point .

[randallwhitman]

ST_Distance accepts any geometry type, so is expected to work for polygon and point. If your coordinates are geographic, then the result is in angular measure. If you need linear distance with geographic coordinates, then Spatial-Framework-for-Hadoop does not do that directly. You can use ST_Distance to get the distance in degrees, then utilize ST_GeodesicLengthWgs84 toward getting the linear distance from the angular distance. See also: http://gis.stackexchange.com/questions/185494/proximity-query-in-hive-using-esri-geospatial-library/185500#185500

[kaxil]

@randallwhitman Hi, thanks for the reply. But the function ST_GeodesicLengthWgs84 accepts a binary datatype or a geometrical point right ?

Whereas the ST_Distance will return a double. So I cannot using something like:

Select ST_GeodesicLengthWGS84(ST_Distance(ST_Point(33.30,42.0),ST_Point(3.0,4.0)))/1000 as
dist, long_text_field_1, long_text_field_2, security_tag, short_text_field, ts
from db1;

[randallwhitman]

Indeed, it's not that straightforward. After getting the angular distance, choose/construct a point at that distance, then make the Line. Did you read through to the linked answer and its references?

[kaxil]

@randallwhitman Yes, I did read it but the problem is that I am not allowed to use API's or code in Java, just need to use hive shell and run queries there.

Thanks for the logic, but again I wonder whether creating a point i.e knowing the co-ordinates of a point at some distance is possible ! Is there a function for the same ?

[randallwhitman]

ST_Point with numerical expressions for the arguments.

[kaxil]

@randallwhitman : To be clear

1) Use ST_Distance to calculate the distance between a Polygon and a Point. The Result would be a numeric value, let say 5 (Angular/Coordinate Distance) 2) Use ST_Point to make a point at that location... ??????? How ??

As ST_Point just takes longitude & latitude as arguments, How can I get the coordinates of that new point at a certain angular distance away.

[kaxil]

@randallwhitman

I tried to follow according to my understanding of what you have described, so what I am doing is

• Calculating distance between a geometry and a point.
• Then using ST_Buffer with the point and distance calculated in the above step to create a polygon with the distance as the radius and the point as the centre.
• Then I find the intersection between the polygon created above and the geometry.
• Then I just create a ST_LineString between the intersecting point & our original point.
• And finally just find distance using ST_GeodesicLengthWGS84

ST_GeodesicLengthWGS84(ST_SetSRID(ST_LineString(array(ST_Intersection(table1.geometry, ST_Buffer(table2.point, ST_Distance(table1.geometry, table2.point) + 1)), table2.point)), 4326))/1000

Also the 1 in ST_Distance(table1.geometry, table2.point) + 1 is used as a buffer.

It worked for me.

Do let me know if I am doing something incorrect. Thanks

[randallwhitman]

Or calculate a ratio based on latitude, as per the above-posted link and its reference links.