02/09/2011

Snapping tolerance: Bit-by-bit, a little goes a long way


Snapping tolerance: Sikit-sikit jadi bukit

Tidak dinafikan bahawa penyediaan peta dapat dilakukan tanpa banyak songeh mengenai QGIS default setting tetapi apabila isu petepatan dihargai, penggunaan snapping option amat berguna. Tetapi soalnya sejauhmana harus ditetapkan unit tolerance? Sini tiadanya pendirian cuma panduan yang boleh  diberi adalah lebih halus unit tolerance, lebih tepatnya akan hasil kerja digitizing. Saya dimaklumkan tolerance yang diamalkan oleh ArcGIS adalah 0.001 dan ada pandangan bahawa range sesuai adalah antara 0.001-0.005 ataupun sekil 1/1,000-1/5,000. Manual QGIS pula menyatakan bagi altitude 300', tolerance yang "reasonably setting" adalah 0.00001 atau 1/10,000 (Apa maksudnya, saya masih berfikir). Argument yang diberi adalah apabila tolerance 0.005 diguna, mana-mana gap tidak dapat dilihat secara kasar (bukan bermakna ianya tidak ada). Cubalah buat sendiri dan setelah beberapa cubaan antara 0.001 and 0.005, saya kira lebih kurang selesa dengan tolerance 0.001 tapi kalau saja nak tunjuk kesan snapping, akan saya pilih 0.01. Aspek kedua snapping option merupakan setting unit tolerance by "map unit" atau "pixel" dimana pilihan pixel ada kaitan dengan saiz monitor yang diguna. Skrin monitor yang mempunyai resolusi tinggi mungkin tidak akan nampak banyak perbezaan snapping tolerance jika unit pixel diguna, maka, bagi kes seperti ini, pilihan map unit merupakan opsyen yang lebih berkesan. Tapi kawan, jika kerja snapping terkena pada node atau vertex yang salah, bolehkah unsnapping dibuat? Tarik node kesana kesini dan kedua-dua vertex tak akan berpisah. Unclick snapping option juga gagal. Jangan panik...solutionnya adalah menu/edit/undo! Tu la...tak cuba tak tahu!

Snapping tolerance: Bit-by-bit, a little goes a long way

There is no denying that the preparation of a plan can be carried out with QGIS with much fuss using the default setting but when the issue of accuracy arises, the use of the snapping option is most useful. But the question of how accurate should the unit of tolerance be? There are no fast rules here just that the more bigger the unit tolerance, the more accurate it will digitizing be. I was told that the tolerance used by ArcGIS is 0.001 and there is an opinion that the suitable range should be between 0.001-0.005 or 1/1,000-1/5,000. The QGIS manual stated at at altitude of 300', a "reasonably setting" tolerance is 0.0001 or 1/10,000 (Whatever that means? I am still wondering) The argument is that when a 0.005 tolerance is used, there is no gap (cannot be visually seen ). I tried it myself and after a few tries between 0.001 and 0.005, I am quite comfortable with 0.001 but to visually it snap, I would choose 0.01. The second aspect of the snapping option is the unit tolerance setting by "map unit" or "pixel unit" whereby the pixel option is relative to the size of the monitor used. The screen monitor with a higher resolution may not show much difference between snapping tolerance if pixel unit is used so in this case, the map unit option is more effective. But friend, if snapping work accidentally hit a wrong node or vertex, can it be undone? Pull the node to the left or right and nothing happens. Unclick the snapping option also fails. Don't panic! The solution is menu/edit/undo. See you never know if you don't try!

2 comments:

  1. en.abbas,
    sy nak tanyer macam mane sy nak coppy line yg sy digitize....atau sy nak copy paralel line yg didigit.
    tq.

    ReplyDelete
    Replies
    1. Dalam edite mode, select single feature/edit/copy feature/paste feature/move feature/save

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