09/09/2012

Cleaning data topology manually





Membaiki topologi data secara manual

Pada latihan QGIS barukini, saya telah melatih cara pengguna QGIS boleh mengeksploitasi modul-modul v.clean yang terdapat pada GRASS GIS. Dari perbincangan yang lanjut, ianya mendedahkan saya kepada kelemahan membaiki topologi data secara automatik oleh GRASS GIS lalu saya melayar Internet dan mendapati Arcinfo juga boleh membaiki topologi data secara automatik seperti di contohi   'Eliminate'. Teknik auto-cleaning oleh Arcinfo dan GRASS GIS sedikit tidak memuaskan kerana adanya kala dimana perisian tidak dapat membaiki kesilapan berasaskan rules yang ianya pegang dan apa yang ternyta di bumi tidak sebegitu yang disangka oleh perisian. Teringin saya menentukan bagaimana QGIS dapat membaiki topologi data secara manual. Hari ini, apa saya mendapati sedia ada merupakan modul 'Check geometry validity' dibawah Vector/Geometry tools, maka, mencubanya. Nampaknya output spatial tidak dapat terpapar seterusnya tetapi perlu dihasilkan dari teks yang ditransformasi dengan bantuan Delimited text layer. Akhirnya, saya berjaya menghasilkan lapisan spatial yang menjelaskan taburan kesilapan mengikut vertex bagi beberapa poligon. Ini adalah baik kerana pengguna boleh terus membaiki vertex tertentu yang memerlukan pindaan.

Cleaning data topology manually

During the recent QGIS training exercise I taught the QGIS user could exploit the many cleaning v.clean modules under GRASS GIS. During discussions, it exposed me to the limitations of auto-cleaning data topology offered by GRASS GIS. I surfed the Internet and found Arcinfo could also auto-clean data topology exemplified by 'Eliminate'. However, the auto-cleaning technique used by both Arcinfo and GRASS GIS left me a bit unsatisfied because there are situations where the software is not able to correctly rectify errors based on rules it held and what is on the ground is not how the software thinks it should be. This spun me to find a way to find out how to get QGIS to clean data topology manually. Today, what I saw currently available is the 'Check geometry validity' tool under Vector/Geometry tools and decided to give it a try. It seems the spatial output could not be immediately shown but had to be transformed from text obtained with the help of the Delimited text layer module. I eventually got a spatial layer showing the distribution of errors by the vertex of many polygons. This is good because the user can now proceed to rectify the specific vertex that needed repair.

4 comments:

  1. En Abbas, maybe dlm ArcInfo kaedah yg lebih sesuai utk rectify adalah gunakan geodatabase topology rules utk rectify area yang ada error & topology rules juga boleh buat autocorrection based on tolerance yang ditentukan oleh user.

    ReplyDelete
    Replies
    1. Saya baca dari Internet, ArcInfo topology rules seperti pada Eliminate adalah lebih kurang sama dengan GRASS GIS (yang QGIS boleh tumpang). Saya setuju sometimes topology rules tak sama on the ground maka repair tak kan tepat. Itu kenapa staf BMGN lebih yakin betulkan error secara manual walaupun banyak. Tolerance is a relative word maka kita boleh bincang samapai maghrib pun tak kan selesai siapa yang lebih tepat.

      Delete
  2. What do you think which one of QGIS and ArcGIS is better in cleaning the topological errors.

    ReplyDelete
    Replies
    1. If you plan on using auto-cleaning features from a particular GIS software you have to question yourself first whether you agree to the rules set by the respective software because factors like acreage is affected where they concern polygons. In any case there are many instances where the user must go manual which is what we do because a GIS software, because neither as in the case of a gap can tell which is the right move i.e. to shift a vertex left or right, up or down to close the gap without supporting data to help justify the move.

      Delete