Manual data verification through QGIS-GRASS

JUPEM cadaster map

Topology error identified

Zoomed-in

Zoomed further in

Verifikasi data secara manual melalui QGIS-GRASS

Saya amat hormati Jab. Pemetaan dan Tanah, Malaysia (JUPEM) sebagai kustodian peta kadaster negara. Bagaimana pun, kini saya mempunyai mainan namanya QGIS dan ingin tahu keberkesanannya bila di integrasikan dengan modul GRASS seperti the Man from U.N.C.L.E. boleh mengubahsuaikan pistol Walter P-38 menjadi senapang dilengkapi telescope. (JUPEM mungkin tidak akan suka membaca blog ini tetapi dilakukan demi ilmu) Pertama, peta kadaster JUPEM bagi negeri Perlis diaktifkan (pilihan ini berasaskan saiz fail) lalu di import ke GRASS. Kemudian, browser GRASS diguna untuk menojolkan topology error lalu dipilin 1 dari 3 output.  Secara random di zoomed-in lalu dipilih 1 garis. Di zoom-in lagi, nampaknya garis bukannya garis tetapi overlap atau gap. Disemak pula dengan peta kadaster asal, nampaknya garis tersebut adalah gap. Pada kawasan lain pula, terdapatnya rectangle. Di zoomed-in dan disemak dengan peta kadaster asal, didapati ianya tidak mempunyai nombor rujukan. Ini adalah gap atau mungkin ada yang lupa mengisikan atribut. Berasaskan pengalaman, kemungkinan besar, opsyen pertama adalah lebih tepat. Apabila di zoomed-out, banyak kes topology error ternyata dan terserah pada anda untuk membuat kesimpulan. Pokoknya, QGIS-GRASS boleh digunakan untuk menonjolkan topology error. Sama ada nak betulkan secara auto atau manual merupakan cerita lain. GRASS mempunyai ciri-ciri auto-cleaning tetapi soalnya adakah anda bersetuju dengan rules yang digunapakai? Ceritanya adalah sama bagi kes proprietari GIS. Sekiranya data cleaning bagi topology error dibuat secara manual jangan terkejut bilangannya boleh sampai ke ribu dan sekiranya saudara tak tahu keadaan sebenar, saya percaya amat sukar membetulkan topology error secara tepat dan melawat tapak tak bermakna garisan yang dilihat pada GIS akan lebih jelas. Buktikan saya salah.

Manual data verification through QGIS-GRASS

I have high respect for Malaysian Land and Survey Dept. (JUPEM) as custodian of Malaysian cadaster maps. Still, I have a toy to play with called QGIS and I wanted to see how it performs when integrated it with GRASS modules just like the Man from U.N.C.L.E. can modify his Walter P-38 pistol into a rifle with its telescopic extension. (JUPEM may not like to read this blog but what I did, I did for the sake of knowledge) Firstly, I activated a JUPEM cadaster map for the state of Perlis (choice was simply base on fail size) then imported it into GRASS. Next, I used the GRASS browser to expose for topology error then chose 1 of 3 outputs. I randomly zoomed-in and selected a line. Zoomed further-in, it was not a line, it was either an overlap or gap. Cross-checked against the original cadaster map, that line appeared to be a gap. Looking elsewhere, there was this rectangle. Zoomed-in and cross-checked with the original cadaster map, it had no reference number. It was either a gap or someone forgot to do his homework. From experience, the first assumption is closer to the truth. Zoomed-out, one can see many cases of topology error, I leave you to make your own conclusion.  The point of this exercise is that it is possible to identify topology errors with QGIS-GRASS. How one goes about cleaning it is another issue: auto or manual. GRASS has auto-cleaning capabilities but are you happy with the rules it applies? This can be said the same for proprietary GIS. If data cleaning of topology error is to be done manually do not be surprised it can run into thousands and unless you know the actual situation, I think it is impossible to accurately rectify the topology error and going to the ground does not mean you will see the lines you saw in GIS will be more clear. Prove me wrong.

Questionable polygon

Adjacent polygon has lot attributes