linestring.is_simple is wrong

[shankari]

Expected behavior and actual behavior.

My non-self-crossing line should return is_simple = True

Steps to reproduce the problem.

import polyline as pl
import shapely as shp

bart_line_pl = 'ly~iV{|ndFqHbHbJqIq@l@vFoFw@t@mCjCdEaEtAaB??NM??lCgCnMgMxG}GtLsL??vAwAdDaD??vCuCb@c@d@g@`@e@\\_@Za@Za@pF}G~@kAz@mAx@kAbAwAhBmCf@w@f@w@f@y@j@y@h@}@l@}@nAeBpAoBhBkC??rAoBzAyB??`EaGnBsCXa@zG{J|Zsn@rK{RzCaG??Vi@??hBsD??HO??R_@??HQ??Zq@zDmI|D}JjA}B~AyBhDgElDiEvGuIbNiP|AaBbBaBdByA`BkApBoAbCuA`CiAhSuIbDyAlCqAhCwAxBoAdBmAzAoAjAiA|@}@~@kAbHmJdAoAfAkArAoAtAmA`BoAfBkA`BeAlGcD`KqFjLsG`BeAxAgArAmArA_BdAwAz@sAxEiIfG{KjAgB~AkBhBgBbBqAnBqArBgAbCmAzC{AfC{AtBcBXUnAkAbBeB`DuDvIuKn@s@n@q@l@m@t@k@r@e@v@e@??fCaB??zAcAzAcA??rA}@rA_Ab@WfJsF??p@c@\\UPMNMNMNMNOJKLOLOJOJOLSJOTe@Ti@^{@??|AoD??JU\\y@Rg@Tq@R{@Ho@Fy@Bm@@o@DkFD{D@mD??@u@???oB@s@Bi@Dw@Hs@??P{@T{@v@gC??Ne@d@_BLe@Jg@Je@Hi@Di@Bk@@s@?q@BcE??BqDBoC@qD?kA?g@Ce@Ec@I_@Kc@Qg@Uc@Wa@Yc@}DeF??yEgG??k@u@??y@gAuEcG??c@i@UYYY]Ya@[a@U_@Sa@Oa@Oc@Oc@Ka@Ic@Gc@Ge@Ee@Cc@Ak@Ae@?oLB??qNBs@?q@Au@?g@Aq@Co@Cq@Em@Gk@Gm@Gi@Ko@Kk@Mm@Om@Qo@Ug@Qk@Ui@Wi@Yi@[g@[g@[c@]c@Y_CgBmBwAGGeBqAmA_Ak@a@e@a@e@a@a@]e@a@e@e@c@e@g@i@??[]_@c@Y]U_@U[W_@S_@S_@Qa@Qa@M_@Ka@Ka@Ic@Ia@??Ko@kBaRw@uF??[cC??YeBUuAYsA[iAc@eA]}@??[i@Wa@Ye@]g@c@k@e@k@m@m@i@i@g@c@e@a@k@a@m@a@m@a@k@]m@[q@]o@[s@Yq@Yu@Yu@Ww@Yw@Uw@Uy@W??{GsBmI}CuMaHsD_CsJuF}l@_]aIkGgG{IqXsf@QYISGOASA_@B[@Ur@ePd@}JpF{nAHeBRsE|@sTDaC??@gACeAKeAOeAWcA]cAe@_Ai@_Ao@{@s@w@{@u@}@s@eE{CkT}OuEgDeYwSwMuJyb@g[mHoFiE}CcAu@kDgCiDeCkBwAiBqA]U]Sa@Qa@O{PyF??ssAmd@kEqAsEgAuE{@_Fu@{Eo@uFo@i|Dm_@_Js@}Ii@{Ia@aIUsIQcKKeLFmLVw~@zD??qI^??ce@xB{BJmBLkBNcBPwAL}AP}AR_LxAw@JkSlCcBVuATuAXqA\\mA\\sBl@yLpDmK~C{Ad@sA^wA^gAVaB`@??yEp@{T|CgBXgB^}A^uA`@qAd@cBl@mBt@gKvD??i@R??{BdAu@X??a@P[L??k@Po@Nk@Ha@Dc@DU@g@Bk@B_@@??e@?e@@c@?g@A_@CWA[EYEQGsAa@kAm@_Au@q@_A??eFuJ??qJqQiQk\\Yg@Ww@Ik@Ac@?c@Dc@Lk@Rg@\\i@`@e@`H}Fb@a@\\c@^k@??Zk@Vm@Rk@Nm@Hg@Fe@Bc@Be@@i@AM?YEi@??Ee@Ea@CMCQEUMi@UeAa@eB??YgAQy@Q{@_@oBGW??QaA??Oy@??O_A??o@iD??}@_F[cB[aBYqA[{Ac@mBa@cBYgAm@eC[oAUaAKo@Ii@??UaB??[cC}@cH??OsA??AG??S}A??YkD??McA??QeB??MkAIu@??Ce@Cc@?g@???m@Bm@??Dg@Hi@Hi@Ni@Ne@??Rg@??Rc@Ta@Ta@Ze@^g@b@i@\\_@|AgBv@_At@_Aj@}@h@cAb@eA\\eAZkARiARuApGsg@NwAHaABY??Bi@D_A???}@A_A??G}AMkA[cB{DgMuCiK}Js^w@}CQiAGu@AeADaA~@cOXmEpAeV^kEr@}I'
ls = shp.geometry.LineString(pl.decode(bart_line_pl)); ls
ls.is_simple

Notes:

• I don't know if this is a generic issue; I only encountered it in this use case.
• If you run this in a notebook, you can display the line ls and confirm that it is not self-crossing (see screenshot)

Operating system

Mac OSX 10.14.5

Shapely version and provenance

Installed from conda-forge

$ conda list shapely
# packages in environment at ...:
#
shapely                   1.6.4            py36he8793f5_0

[shankari]

Uploading screenshot with scaling to show the string better.

[sgillies]

@shankari green means only that the geometry is "valid", which is always the case for line strings. I suspect that your line really does self-intersect at some point, even if that is only due to floating point precision errors.

It would probably be good to make linestrings red if they are not simple, yes? Instead of coloring based on is_valid.

[shankari]

@sgillies I didn't even know that linestring color represented validity 😃 I just put that in to show that there are no self-intersections.

As you can probably tell, that is a public transit line and when I display it on a map, I can zoom and pan. And I don't see any self-intersections or anything that might be close to a self-intersection.

Is there a way I can determine the supposed intersection point so that I can check that location?

[kannes]

You can compare the coordinates of the linestring pair-wise:

for a, b in zip(ls.coords, ls.coords[1:]): 
    if a == b: 
        print(a, b) 

You have many duplicate coordinates, which is not allowed in the Simple Features specification.

A possible automatic fix would ls.simplify(0).

[sgillies]

@kannes thank you 🙏

[kannes]

Right back at you! Any time I file a bug report or voice a wish, I also look at issues to see if I can help, that's the spirit.