27/09/2012

Snapping option issues


Snapping option issues

Since my department now uses all 3 GIS softwares, I decided to compare notes between ArcGIS, Mapinfo and QGIS in anticipation of queries. Today, I looked at snapping tolerance and found out that both ArcGIS and MapInfo snapping unit are only available in pixels whereas in QGIS, user can opt between pixels or mapunit have. (I am informed by blog member Saya Suka GIS below that this is not quite true for ArcGIS) Then again, the QGIS User Guide states that "the advantage of choosing pixels is that the snapping tolerance doesn't have to be changed after zoom operations". Next, I found out that the default snapping tolerance of ArcGIS and MapInfo is respectively 3 and 5 pixels whereas for QGIS, it is not set at 0.000000 either in pixel or mapunit. This means that the moment a ArcGIS and MapInfo layer is created, snapping of a digital feature is already possible and is comparatively more accurate in ArcGIS than in MapInfo by a ratio of 3:5. This does not exist in QGIS and if the user wants snapping, then customize a preferred tolerance. So what is the de facto set tolerance? Well, the more zeros there are, the more accurate the layer becomes where 0.001 has a higher tolerance than 0.1 but that level depends on the user's preference, for example, I was told that the Malaysian Land and Survey Dept. (JUPEM) for cartography purpose uses a snapping tolerance of 0.000001 pixels (you have to confirm it from the horse's mouth). This means that if a value-added layer was made from JUPEM's cadaster map, any amendment concerning topology ought to follow the snapping tolerance of the custodian to ensure that accuracy is maintained. No one is said another agency cannot use  a different snapping tolerance level but there are pros and cons to consider. My department has yet to establish a department snapping tolerance (policy) for all town planning agencies and I feel there should be and hope to address this issue in future. Thirdly, I also found out that a layer created with QGIS default tolerance of 0.000000 pixels when activated as a .tab file by MapInfo did not show pixel tolerance of the earlier GIS software but MapInfo's 5 pixel. This means another layer prepared at a tolerance level of say 3 pixel, if uploaded into MapInfo, unless adjusted, will stay 5 pixels. Looks like there is a need for standardization and coordination on this issue.

7 comments:

  1. En Abbas, just for info...ArcGIS snapping unit are also available in mapunit. It can be found in Editor > Snapping > Options in ArcGIS10.

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    1. Thanks, I will tell my staff because he told me ArcGIS only delivers in pixels, myself, not a ArcGIS user.

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  2. Sorry for wrong information En Abas,i'am still learning and not a arcGIS "tegar" user..hehe..

    now i want to share how to active selection value for snapping. 1st u must active "use classic snapping" option at (editor-Option- General and tick at "use classic snapping".

    Go to (Editor > Snapping > Options)then after that u can see selection menu for snapping tolerance measurement unit between "pixel" or "map unit".

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    1. Yes, Thank you. We too JUST found that it is under editor/option/general/use classic option/ok because editor/snapping/option did not produce the required result.

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    2. yes...must click "use classic snapping" first if using version10.

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  3. salam tuan,
    melalui pembacaan dan rujukan saya di web tuan boleh dikatakan pelopor penggunaan OSSGIS di malaysia terutamanya sektor awam.
    cuma timbul satu persoaalan sejauh mana keupayaan qgis ini dalam melakukan proses transformasi data
    misalnya
    data daripada WGS84 kepada RSO
    tuan pernah buat tak pertukaran koordinat ini .
    melalui eksperimen yang saya jalankan hasilnya agak menghampakan..
    WGS84 : Lat.long ; 3.86 , 103.31
    hasilnya adalah (QGIS): 922543, -15292.07
    sepatutnya (rujukan JUPEM): 590702.42, 427162.02
    daripada perbandingan ini menunjukan amat besar bezanya.
    satu lagi sekiranya saya ada data daripda shapefile RSO dengan menggunakan QGIS ini saya hendak membuat translasi data kepada KML dan dibuka kedalam Google earth hasilnya juga menghampakan jauh masuk ke dasar laut..

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    1. >sejauh mana keupayaan qgis ini dalam melakukan proses transformasi data?

      Saya rasa isu disini bukan isu perisian QGIS tetapi isu parameter. Setahu saya, QGIS developers ada menyediakan parameter yang mereka fikir sesuai untuk membolehkan conversion tetapi QGIS menyatakan dalam QGIS USER GUIDE sekiranya pengguna kurang puashati dengan parameter tersebut, no problem, mereka telah mewujudkan satu custom CRS dibawah Menu/Setting/Custom CRS dimana pengguna boleh input parameter tertentu kedalam Custom CRS. In fact, parameter conversion boleh dibeli dari JUPEM maka terserah kepada pengguna untuk mengambil tindakan selanjutnya. Saya sendiri tidak pernah guna Custom CRS tersebut so tak dapat mendorong dalam hal ini.Maaf.

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