13/09/2011

Tips from QGIS User Guide


Tips dari Panduan Pengguna QGIS

Mengikut ramai penjual telefon mobil yang telah saya bersembang, kata mereka ramai pembeli tidak suka merujuk pada Panduan Pengguna setelah membeli produk masing-masing walhal banyak persoalan yang dibangkitkan mengenai produk tersebut boleh dijawab dengan mudah jika pengguna membaca Panduan Pengguna setelah membeli produk. Mendengar nasihat ini, terus saya mengambil initiatif untuk membaca sekali lagi Panduan Pengguna Quantum GIS Versi 1.5 yang telah saya hardcopy-kan tetapi kali ini secara terperinci.  Antara perkara-perkara menarik yang saya terjumpa adalah termasuk:

  • Definition of GIS
    Setelah beberapa tahun ni, saya sendiri tak sangka :-( GIS is not a software but "a collection of software that allows you to create, vizualize, query and analyse geospatial data" (Ref: Chapter 1.1 Introduction to GIS)

  • Indexing improves performance
    Hebatnya perkara ni yang dinamakan spatial indexing. "A spatial indexing will improve the speed of both zooming and panning". (Ref:3.1.2 Improving Performance) Here, QGIS uses a .qix extension. But while a user may be reluctant to use .qix extensions , importing the file into GRASS also creates spatial indexing and improves performance but further to that data topology is cleansed.

  • Benefits of using PostGIS"
    The benefits of PostGIS are the spatial indexing, filtering and query capabilities it provides. Using PostGIS, vector functions such as select and identify work more accurately than with OGR layers in QGIS. (Ref: 3.2 PostGIS Layers)

  • Improving Performance
    "You can improve the drawing performance of PostgrSQL layers by ensuring that a spatial index exists on each layer in the database" (Ref:3.2.5 Improving Performance)

  • Snapping tolerance
    " Your results may vary but something on the order of 300ft should be fine at a scale of 1:10,000 should be a reasonable setting (Ref: 3.5.1 Setting the Snapping Tolerance and Search Radius) This works out at 0.00001 and I am left wondering if the relative size of a study area has a direct effect to what tolerance is "reasonable" e.g. in a country with a huge span of area like USA may feel the need for 0.00001 whereas a small country like Malaysia the need for 0.0001 is "reasonable" enough.
  • Interoperable
    Interoperable (ref: 5.1 What is OGC Data?) This seems to be the coin (a) phrase of the future where data sharing is concerned and with that in mind, whatever the Open Geospatial Consortium (OGC) recommends for the open source community looks like the direction of the future.

    Tips from QGIS User Guide


    From many mobile telephone retailers whom I talked to, they say that many buyers do not like to refer to the User Manual once the respective product was purchased but in fact many queries could be answered had users read the User Manual after buying the product. Hearing this advice, I took the initiative to read again this time in detail, the Quantum GIS Versi 1.5 user Guide having hardcopied it.  Among the points I found include:

    • Definition of GIS
      After many years, I did not realize :-( GIS is not software but "a collection of software that allows you to create, vizualize, query and analyse geospatial data" (Ref: Chapter 1.1 Introduction to GIS)

    • Indexing improves performance
      This thing called spatial indexing is terrific. "A spatial indexing will improve the speed of both zooming and panning". (Ref:3.1.2 Improving Performance) Here, QGIS uses a .qix extension. But while a user may be reluctant to use .qix extensions , importing the file into GRASS also creates spatial indexing and improves performance but further to that data topology is cleansed.

    • Benefits of using PostGIS"
      The benefits of PostGIS are the spatial indexing, filtering and query capabilities it provides. Using PostGIS, vector functions such as select and identify work more accurately than with OGR layers in QGIS. (Ref: 3.2 PostGIS Layers)

    • Improving Performance
      "You can improve the drawing performance of PostgrSQL layers by ensuring that a spatial index exists on each layer in the database" (Ref:3.2.5 Improving Performance)

    • Snapping tolerance
      " Your results may vary but something on the order of 300ft should be fine at a scale of 1:10,000 should be a reasonable setting (Ref: 3.5.1 Setting the Snapping Tolerance and Search Radius) This works out at 0.00001 and I am left wondering if the relative size of a study area has a direct effect to what tolerance is "reasonable" e.g. in a country with a huge span of area like USA may feel the need for 0.00001 whereas a small country like Malaysia the need for 0.0001 is "reasonable" enough.

    • Interoperable
      Interoperable (ref: 5.1 What is OGC Data?) This seems to be the coin (a) phrase of the future where data sharing is concerned and with that in mind, whatever the Open Geospatial Consortium (OGC) recommends for the open source community looks like the direction of the future.

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