Store circle in MySQL GeoSpatial Database

Question

I want to be able to store a circle using a fixed point g and radius d, then get those values back when retrieving the information.

The only way I've found to use those arguments to create a geographic object is to use buffer which produces a polygon:

https://dev.mysql.com/doc/refman/5.6/en/spatial-operator-functions.html#function_buffer

SELECT ASTEXT( BUFFER ( POINT( 10, 10 ), 5 ) );

| POLYGON((10 0,9.50932325672582 0.012045437948275506,9.019828596704395 0.048152733278032045,8.532695255446383 0.10823490035219052,8.049096779838717 0.1921471959676957,7.570198200967361 0.2996874680545609,7.097153227455378 0.4305966426779104,6.631101466077799 0.5845593481697922,6.173165676349102 0.7612046748871322,5.724449065697179 0.9601070687655664,5.286032631740024 1.1807873565164506,4.858972558067784 1.4227138999972784,4.444297669803978 1.6853038769745474,4.043006955075667 1.9679246851935517,3.6560671583635447 2.2698954663726303,3.2844104515298165 2.5904887464504087,2.9289321881345254 2.9289321881345254,2.5904887464504087 3.2844104515298165,2.2698954663726303 3.6560671583635447,1.9679246851935517 4.043006955075667,1.6853038769745474 4.444297669803978,1.4227138999972784 4.858972558067784,1.1807873565164506 5.286032631740024,0.9601070687655664 5.724449065697179,0.7612046748871322 6.173165676349102,0.5845593481697922 6.631101466077799,0.4305966426779104 7.097153227455378,0.2996874680545609 7.570198200967361,0.1921471959676957 8.049096779838717,0.10823490035219052 8.532695255446383,0.048152733278032045 9.019828596704395,0.012045437948275506 9.50932325672582,0 10,0.048152733278032045 10.980171403295605,0.10823490035219052 11.467304744553617,0.1921471959676957 11.950903220161283,0.2996874680545609 12.429801799032639,0.4305966426779104 12.902846772544622,0.5845593481697922 13.368898533922202,0.7612046748871322 13.826834323650898,0.9601070687655664 14.27555093430282,1.1807873565164506 14.713967368259976,1.4227138999972784 15.141027441932216,1.6853038769745474 15.555702330196022,1.9679246851935517 15.956993044924333,2.2698954663726303 16.343932841636455,2.5904887464504087 16.715589548470184,2.9289321881345254 17.071067811865476,3.2844104515298165 17.409511253549592,3.6560671583635447 17.73010453362737,4.043006955075667 18.03207531480645,4.444297669803978 18.314696123025453,4.858972558067784 18.577286100002723,5.286032631740024 18.81921264348355,5.724449065697179 19.039892931234434,6.173165676349102 19.238795325112868,6.631101466077799 19.41544065183021,7.097153227455378 19.569403357322088,7.570198200967361 19.70031253194544,8.049096779838717 19.807852804032304,8.532695255446383 19.89176509964781,9.019828596704395 19.95184726672197,9.50932325672582 19.987954562051726,10 20,10.49067674327418 19.987954562051726,10.980171403295605 19.95184726672197,11.467304744553617 19.89176509964781,11.950903220161283 19.807852804032304,12.429801799032639 19.70031253194544,12.902846772544622 19.569403357322088,13.368898533922202 19.41544065183021,13.826834323650898 19.238795325112868,14.27555093430282 19.039892931234434,14.713967368259976 18.81921264348355,15.141027441932216 18.577286100002723,15.555702330196022 18.314696123025453,15.956993044924333 18.03207531480645,16.343932841636455 17.73010453362737,16.715589548470184 17.409511253549592,17.071067811865476 17.071067811865476,17.409511253549592 16.715589548470184,17.73010453362737 16.343932841636455,18.03207531480645 15.956993044924333,18.314696123025453 15.555702330196022,18.577286100002723 15.141027441932216,18.81921264348355 14.713967368259976,19.039892931234434 14.27555093430282,19.238795325112868 13.826834323650898,19.41544065183021 13.368898533922202,19.569403357322088 12.902846772544622,19.70031253194544 12.429801799032639,19.807852804032304 11.950903220161283,19.89176509964781 11.467304744553617,19.95184726672197 10.980171403295605,19.987954562051726 10.49067674327418,20 10,19.95184726672197 9.019828596704395,19.89176509964781 8.532695255446383,19.807852804032304 8.049096779838717,19.70031253194544 7.570198200967361,19.569403357322088 7.097153227455378,19.41544065183021 6.631101466077799,19.238795325112868 6.173165676349102,19.039892931234434 5.724449065697179,18.81921264348355 5.286032631740024,18.577286100002723 4.858972558067784,18.314696123025453 4.444297669803978,18.03207531480645 4.043006955075667,17.73010453362737 3.6560671583635447,17.409511253549592 3.2844104515298165,17.071067811865476 2.9289321881345254,16.715589548470184 2.5904887464504087,16.343932841636455 2.2698954663726303,15.956993044924333 1.9679246851935517,15.555702330196022 1.6853038769745474,15.141027441932216 1.4227138999972784,14.713967368259976 1.1807873565164506,14.27555093430282 0.9601070687655664,13.826834323650898 0.7612046748871322,13.368898533922202 0.5845593481697922,12.902846772544622 0.4305966426779104,12.429801799032639 0.2996874680545609,11.950903220161283 0.1921471959676957,11.467304744553617 0.10823490035219052,10.980171403295605 0.048152733278032045,10.49067674327418 0.012045437948275506,10 0)) |

1 row in set (0.00 sec)

My problem with this is that I cannot retrieve the point and radius when selecting this row in the future, instead I get the polygon back.

Is there not a better way to store a circle for use with MySQL GeoSpatial Extensions?

Answer 1

Spatial databases are not great at storing curves; rather they tend to be approximated by lots of straight segments, so you're not actually retaining a circle. Support for curved geometries is improving but it's still not really there.

I would probably store the geometry as you have, as well as the radius as a floating point value.

Alternatively, if you only want to store the geometry, you can obtain the centre of the polygon ("circle") with the Centroid function, and then get the radius by converting the polygon to a line and determining the distance between the centroid and the line. There are other ways to determine this distance, too. the caveat is that because this is only an approximation of a circle, the distance between the centre of the circle and its edge is different when measured between the centre and one of the vertices, and when measured from the centre and an edge between two vertices. So if you do not store the radius independently of the geometry, ideally you should measure distance between a vertex and the centre, which will be equal to the original radius (buffer distance). Practically the difference will be rather small so long as your "circle" has dense vertices.