[postgis-users] Problem with overlay

Thu Jul 5 11:25:25 PDT 2007

This confirms that it's a precision issue.  The endpoints of the second 
line lie extremely close to segments of the first line, but are NOT 
exactly coincident with them.  (As evidence of this, when I computed the 
distance between the first endpoint and the first line, I got a value 
of  2.984496438840695E-10 m. Of course, this figure itself is imprecise 
and subject to roundoff error, but it gives the idea.)

You can't really use overlaps() to determine if these lines are 
coincident.  You'll need to use buffering to determine if the second 
line is no further away than a given tolerance from the first.

Jonathan Aguero wrote:
> Martin,
> Here it is:
> For the road segment 
> "SRID=2927;LINESTRING(2409426.45248153 864440.726373002,2409646.552
> 864450.813,2409724.132 864459.6,2409866.429 864498.596,2410149.525
> 864603.864,2410692.582 864788.24,2410976.74738379 864882.576623256)"
>
> For the curve
> "SRID=2927;LINESTRING(2409687.92785203 864455.49938324,2409724.132
> 864459.6,2409841.01853063 864491.63234888)"
>
>
> Jonathan Aguero Valverde
> Research Assistant and Ph.D. Candidate
> Pennsylvania State University
> The Pennsylvania Transportation Institute
> 201 Transportation Research Building
> University Park, PA 16802
> www.personal.psu.edu/jua130
>
> -----Original Message-----
> From: postgis-users-bounces at postgis.refractions.net
> [mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Martin
> Davis
> Sent: Wednesday, July 04, 2007 2:02 PM
> To: PostGIS Users Discussion
> Subject: Re: [postgis-users] Problem with overlay
>
> Can you post the actual WKT for the result linestrings?  I don't have a 
> PostGIS instance handy to run the SQL.
>
> I'm betting that it is a precision issue.  One way to handle this would 
> be to use a buffer around one of the geometries you want to compare, to 
> "fuzz" the comparison.
>
> Jonathan Aguero wrote:
>   
>> Martin, 
>>
>> Here it is the original LINESTRINGM and the two geometries created from it
>> with the locate_between_measures function. In theory, their intersection
>>     
> is
>   
>> a linestring but I might be experiencing the coordinate drift you
>>     
> mentioned.
>   
>> Now, as I see it, this is a precision problem, not a problem with my
>> definitions of the geometries. What do you think? 
>>
>> CREATE TABLE lrs2 (gid int4, road_inv varchar(4)); 
>> SELECT AddGeometryColumn('lrs2', 'the_geom', 2927, 'LINESTRINGM', 3);
>> INSERT INTO LRS2 (gid,road_inv, the_geom) VALUES
>> (1, '290', GeomFromText('LINESTRINGM(2406383.283 861534.851
>>     
> 0.07,2406359.112
>   
>> 861586.107 0.082,2406335.853 861620.089 0.091,2406310.002 861656.69
>> 0.101,2406255.656 861722.092 0.12,2406212.863 861767.807 0.133,2406167.562
>> 861808.508 0.147,2405858.946 862103.134 0.24,2405700.764 862300.136
>> 0.26,2405597.455 862454.556 0.313,2405561.594 862504.091 0.33,2405526.644
>> 862606.293 0.35,2405376.043 863080.764 0.444,2405107.283 863988.701
>> 0.622,2405089.364 864074.548 0.639,2405083.673 864156.245
>>     
> 0.654,2405087.106
>   
>> 864240.691 0.67,2405115.272 864242.612 0.675,2405814.858 864282.694
>> 0.8,2406187.349 864301.081 0.869,2407373.155 864345.473 1.09,2408850.001
>> 864414.309 1.37,2409646.552 864450.813 1.522,2409724.132 864459.6
>> 1.537,2409866.429 864498.596 1.565,2410149.525 864603.864
>>     
> 1.623,2410692.582
>   
>> 864788.24 1.733,2411535.108 865067.94 1.902,2411877.4 865184.391
>> 1.971,2411997.555 865229.746 1.996,2412066.702 865258.947 2.01
>>     
> ,2412122.725
>   
>> 865291.502 2.022,2412189.604 865347.555 2.039,2413099.432 866096.864
>> 2.264,2413233.115 866208.772 2.297,2413337.133 866308.421
>>     
> 2.325,2413649.134
>   
>> 866597.512 2.406,2414104.962 867058.318 2.53,2414174.266 867117.086
>> 2.547,2414329.433 867259.172 2.586,2414541.968 867377.955
>>     
> 2.631,2415561.425
>   
>> 867898.663 2.843,2415663.666 867950.203 2.865,2415711.381 867964.733
>> 2.874,2415760.712 867974.327 2.883,2415834.682 867983.789 2.897,2415954.61
>> 867986.436 2.919,2417079.882 868026.214 3.128,2417170.24 868029.877
>> 3.145,2417195.169 868042.176 3.15,2417721.664 868392.441 3.27,2417796.276
>> 868423.849 3.285,2418197.637 868559.305 3.366,2418832.192 868788.621
>> 3.495,2419543.831 869053.946 3.641,2419770.891 869135.697
>>     
> 3.687,2419997.995
>   
>> 869225.542 3.733,2421265.462 869754.998 3.996,2421387.091 869796.842
>> 4.021,2421476.281 869826.718 4.038,2421571.488 869846.445
>>     
> 4.057,2421664.639
>   
>> 869860.112 4.075,2421870.976 869869.151 4.115,2422422.273 869893.302
>> 4.22,2423118.185 869912.227 4.351,2423322.658 869910.882 4.39,2423543.675
>> 869887.113 4.432,2423761.002 869840.387 4.474,2423900.347 869825.931
>> 4.5,2425047.751 869848.138 4.717,2425262.25 869851.04 4.757,2425349.276
>> 869854.623 4.773,2425426.24 869868.376 4.788,2425559.956 869900.037
>> 4.814,2425663.321 869931.861 4.834,2425807.25 869981.679 4.863 ,2427651.83
>> 870598.722 5.23,2428262.085 870816.397 5.353,2430198.032 871483.911
>> 5.74,2431505.587 871940.293 6,2432815.135 872390.593 6.26,2435415.059
>> 873293.352 6.78,2437780.605 874089.52 7.257,2437820.359 874103.312 
>> 7.265,2438045.344 874181.343 7.31,2438366.008 874269.892 7.373,2438966.15
>> 874475.042 7.494,2440234.314 874916.234 7.749,2442988.007 875863.599
>> 8.303,2443271.918 875961.321 8.36,2443560.018 876049.762 8.416,2443608.255
>>     
>
>   
>> 876068.505 8.426,2443666.681 876097.327 8.438,2443732.747 876133.708
>> 8.452,2443788.707 876175.211 8.465,2444125.925 876459.689
>>     
> 8.546,2444432.057
>   
>> 876711.694 8.62,2444631.05 876873.741 8.672,2444787.543 877000.844 
>> 8.712,2444943.989 877119.08 8.751,2445073.637 877200.723 8.782,2445208.325
>> 877277.273 8.813,2445344.218 877342.415 8.843,2445453.449 877396.299
>> 8.868,2445811.841 877539.109 8.945,2445825.476 877557.714 8.95,2445938.059
>>     
>
>   
>> 877587.195 8.972,2446085.112 877622.033 9,2446299.084 877660.47
>> 9.04,2446608.007 877688.501 9.098,2447923.128 877847.699 9.346,2448425.232
>> 877912.299 9.44,2452388.897 878388.295 10.2,2452678.1 878436.029
>> 10.26 ,2452785.541 878453.306 10.28,2452839.486 878461.985
>>     
> 10.29,2452936.212
>   
>> 878477.791 10.309,2453147.417 878512.326 10.35,2453203.975 878520.16
>> 10.361,2453529.509 878565.275 10.424,2453718.67 878587.397
>>     
> 10.46,2454143.639
>   
>> 878624.842 10.541,2454592.191 878672.24 10.628,2454813.568 878695.375
>> 10.67,2455810.625 878816.796 10.861,2456783.057 878923.353
>> 11.046,2457948.075 879068.144 11.269,2459000.96 879187.493
>>     
> 11.47,2459503.893
>   
>> 879234.27 11.566,2459785.345 879264.009 11.619,2460056.408 879299.012
>> 11.671,2460426.624 879367.34 11.743,2460854.335 879471.822
>> 11.826,2461219.594 879587.06 11.899,2462427.66 879986.859
>>     
> 12.14,2463875.818
>   
>> 880473.917 12.43 ,2464522.953 880697.024 12.558,2465634.74 881112.965
>> 12.78,2466133.525 881277.739 12.86,2466464.64 881418.43 12.931,2468377.709
>> 882145.196 13.333,2469223.238 882450.574 13.51,2470998.744 883136.062
>> 13.871,2471904.723 883482.827 14.055,2472282.975 883623.285
>> 14.132,2474415.388 884367.481 14.56,2476883.851 885294.585
>> 15.057,2477619.048 885577.557 15.205,2478111.881 885802.791
>> 15.307,2478588.523 886044.39 15.408,2479003.256 886276.557 
>> 15.497,2479643.615 886689.823 15.641,2479954.098 886909.531
>> 15.713,2482447.575 888637.839 16.284,2483535.9 889406.806
>>     
> 16.535,2484758.36
>   
>> 890295.507 16.82,2485433.873 890812.227 16.982,2486278.785 891394.59
>> 17.177 ,2486869.777 891816.656 17.315,2487564.761 892296.149
>> 17.475,2488317.986 892821.017 17.649,2489120.621 893384.276
>> 17.836,2489836.725 893868.926 18,2491205.446 894830.469 18.31)', 2927
>> ));
>>
>> CREATE TABLE road_segments (road_id int4, road_inv varchar(4),begmp real, 
>> endmp real);
>> SELECT AddGeometryColumn('road_segments', 'the_geom', 2927, 'LINESTRING',
>> 2);
>> INSERT INTO road_segments (road_id,road_inv, begmp, endmp) VALUES
>> (10, '290', 1.48 , 1.79);
>> UPDATE road_segments SET the_geom =
>> force_2d(locate_between_measures(lrs2.the_geom,begmp,endmp))
>> FROM lrs2;
>>
>> CREATE TABLE curves (curve_id int4, road_inv varchar(4),begmp real, endmp
>> real);
>> SELECT AddGeometryColumn('curves', 'the_geom', 2927, 'LINESTRING', 2); 
>> INSERT INTO curves (curve_id,road_inv, begmp, endmp) VALUES
>> (100, '290', 1.48, 1.79);
>> UPDATE curves SET the_geom =
>> force_2d(locate_between_measures(lrs2.the_geom,begmp,endmp))
>> FROM lrs2;
>>
>> SELECT road_id FROM road_segments, curves 
>> WHERE road_segments.the_geom && curves.the_geom AND
>> overlaps(road_segments.the_geom, curves.the_geom);
>>
>>
>> Jonathan Aguero Valverde
>> Research Assistant and Ph.D. Candidate
>> Pennsylvania State University
>> The Pennsylvania Transportation Institute
>> 201 Transportation Research Building
>> University Park, PA 16802
>> www.personal.psu.edu/jua130
>>
>> -----Original Message-----
>> From: postgis-users-bounces at postgis.refractions.net
>> [mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Martin
>> Davis
>> Sent: Tuesday, July 03, 2007 8:07 PM
>> To: PostGIS Users Discussion
>> Subject: Re: [postgis-users] Problem with overlay
>>
>> Can you post the WKT of two linestrings which you think should overlap?
>>
>> Overlaps has a very narrow definition. For Linestrings, two linestrings 
>> overlap if and only if their intersection is itself a linestring (i.e. 
>> they overlap in a point-set of dimension 1).
>>
>> I suspect you may be seeing the result of the inevitable coordinate 
>> drift caused by computing a new coordinate along a line segment. In 
>> almost all cases, doing this results in two new line segments which are 
>> NOT coincident with the original.
>>
>> Jonathan Aguero wrote:
>>   
>>     
>>> Rhys,
>>>
>>> That's the point, they should /overlap/ since they were created from 
>>> the same linear reference system and the road segment '/contains/' the 
>>> curve.
>>>
>>> I also tried with /intersects/ but the problem is that it picks up 
>>> more segments than I need. For example if a curve ends at the same 
>>> point than a road segment; theoretically, the overlaps should pick up 
>>> only that segment while /intersects/ pick also the following segment 
>>> that starts when the previous segment ends. Think also in a situation 
>>> where a road intersection is located in a curve: /intersects /will 
>>> select the other segments that converge in the intersection even 
>>> though they have no curves.
>>>
>>> Now, I might be misinterpreting what the overlap should do; if this is 
>>> the case please let me know.
>>>
>>> Thanks.
>>>
>>>     
>>>       
>>   
>>     
>
>   

-- 
Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022