Oracle Spatial function to find nearest line string based on lat/long

Question

Hi,
Here is my scenario. I have a table that contains geometries of type line strings (the roadway network). The line geomteries are of type Ohio state plane south (SRID 41104).
I have a requirement - given a lat/long, find the line string that snaps to that lat/long or the nearest set of line strings within a distance of 0.02 miles.
This is a typical example of trying to identify a crash location on our roadway network. The crashes being reported to us in lat/long thru the GPS system.
How can i acheive this through any spatial functions?
Thanks for the help in advance.
thanx,
L.

Answer 1

Hi L,
That is not the way I would do it. I would convert my road segments to LRS data, then you can do all queries on the same data.
Or, if you do not want to modify your original data, create a copy of your road segments with the same ID's and convert the copy into LRS data. If you keep the ID's identical, you can easily use geometry from one and LRS data from the other - as long as you are sure the ID is the same.
Which will make the workflow a bit easier:
1. Use SDO_NN to get the closest segments
2. Use SDO_LRS.PROJECT_PT to get the projected point
3. Use SDO_LRS.GET_MEASURE to get the measure
And most of these you can incorporate into one single query. Now I am writing this of the top of my head (It's been a while since I played with LRS). so this has not been tested, but something like this should work (but could probably be greatly improved - it's getting late for me :-) ):
SELECT
SDO_LRS.FIND_MEASURE  --//find_measure needs an LRS segment and a point
    SELECT            --//here we select the LRS segment
      r.geometry 
    FROM
      roadsegments r
    WHERE SDO_NN(r.geometry,    --//based on the given GPS point
                 sdo_geometry(2001, 41104, sdo_point_type(lat,lon,NULL), NULL, NULL), 
                 'sdo_num_res=2 distance=0.02 unit=mile') = 'TRUE'
  SDO_LRS.PROJECT_PT  --//We project the point on the LRS segment
      SELECT         --//here we select the LRS segment (again, which could probably be improved!!)
        r.geometry 
      FROM
        roadsegments r
      WHERE SDO_NN(r.geometry,
                   sdo_geometry(2001, 41104, sdo_point_type(lat,lon,NULL), NULL, NULL), 
                   'sdo_num_res=2 distance=0.02 unit=mile') = 'TRUE'
    sdo_geometry(2001, 41104, sdo_point_type(lat,lon,NULL), NULL, NULL) --//The GPS point again
AS milemarker from dual;So it is not as complicated as you think, it can easily be done with just one query (SQL can do a lot more than you think ;-) ).
Good luck,
Stefan