Nambahin sedikit dari mas Noor, saya kutip dari buku GIS Mudah-mudahan bisa membantu .....8-)
SPHERE & SPHEROID 'The shape and size of a geographic coordinate system's surface is defined by a sphere or spheroid. Although the earth is best represented by a spheroid, the earth is sometimes treated as a sphere to make mathematical calculations easier. The assumption that the earth is a sphere is possible for small-scale maps, those smaller than 1:5,000,000. At this scale, the difference between a sphere and a spheroid is not detectable on a map. However, to maintain accuracy for larger-scale maps (scales of 1:1,000,000 or larger), a spheroid is necessary to represent the shape of the earth. ' "A sphere is based on a circle, while a spheroid (or ellipsoid) is based on an ellipse. The shape of an ellipse is defined by two radii. The longer radius is called the semimajor axis, and the shorter radius is called the semiminor axis." DATUM " While a spheroid approximates the shape of the earth, a datum defines the position of the spheroid relative to the center of the earth. A datum provides a frame of reference for measuring locations on the surface of the earth. It defines the origin and orientation of latitude and longitude lines. Whenever you change the datum, or more correctly, the geographic coordinate system, the coordinate values of your data will change. " salam rahmat -----Original Message----- From: SYARIFUDDIN Noor [mailto:[EMAIL PROTECTED] Sent: Monday, June 16, 2003 6:14 PM To: [EMAIL PROTECTED] Subject: RE: [iagi-net-l] Lagi lagi ... sistim proyeksi Untuk proyeksi peta, sebenarnya tergantung seberapa luas daerah yang mo dipetakan, kalau masih dibawah 6 derajat, maka bisa pakai standard UTM (zone 47 sampai 54, tinggal pilih yang paling sesuai). Kalau daerahnya terletak di perbatasan zona yang standard, maka musti bikin non-standard UTM dengan central meridian (CM) yang diplih sendiri. Nah kalau mo bikin peta sak Indonesia, tentunya UTM kurang pas, jadi orang bisa pake Mercator (MR) dan bisa yang pakai CM-nya Jakarta (106 lebih dikit) atau yang betul-betul tengahnya Indonesia (117). Kalau di bawah 10,000 skalanya ya sudah masuk skala detail, musti pake sistem lokal yang diikat ke titik geodesi terdekat. Ada 3 term dalam sistem geodesi yang kayaknya sering kecampur-campur: - spheroid : Bessel, GRS-67, Everest Modified, Hayford, WGS72, WGS84 etc - proyeksi : UTM, Mercator etc - datum : Batavia, Samboja, Kertau, Genuk, etc Kecuali WGS84, semua spheroid yang lain umumnya juga mencantumkan datum yang dipakai karena datumnya tidak universal seperti pada WGS84. salam, "Harahap, Rahmat RH" <[EMAIL PROTECTED]> 13/06/2003 01:35 PM Please respond to iagi-net To: [EMAIL PROTECTED] cc: Subject: RE: [iagi-net-l] Lagi lagi ... sistim proyeksi Ikut nimbrung, Yang standar sih UTM dengan datum WGS84 untuk skala 25.000 ke atas. Kalau 10.000 dan skala besar mungkin lebih praktis proyeksinya dengan lokal sistem/jaring geodesi yang ada di daerah tersebut. Tapi ini juga tergantung dari kebutuhan si pengguna. Sumatra dulu pakai Batavia/Jakarta tapi sudah ke WGS 84. Kalimantan kalau nggak salah Segora, atau Samboja untuk Kal Tim. Indonesia Timur saya kurang tahu. Mungkin yang dari freeport lebih tahu. tulisan mengenai proyeksi dan datum ini ada (pdf) hanya nggak bisa kirim kalau ke Iagi net (no attachment) rahmat -----Original Message----- From: Bambang Murti [mailto:[EMAIL PROTECTED] Sent: Friday, June 13, 2003 2:14 PM To: Iagi-Net (E-mail) Subject: [iagi-net-l] Lagi lagi ... sistim proyeksi Ini mumpung tahu bahwa ada geodetis yang "suka nimbrung" disiko ... :) (Lha njenengan itu "detis" atau "logis" sih mas Aria ? Untuk daerah deket-deket katulistiwa, jelas salah satunya adalah Indo, kira-kira sistim proyeksi dan spheroid yang paling sesuai apa ya ? Katakanlah skala yang lazim dipakai, 1:10.000, 1:25.000, 1:50.000 dan 1:100.000 Ada ndak ya yang punya list-nya, ndak usah jauhjauh, di belantara migas dulu aja deh... Bambang --------------------------------------------------------------------- To unsubscribe, e-mail: [EMAIL PROTECTED] Visit IAGI Website: http://iagi.or.id IAGI-net Archive 1: http://www.mail-archive.com/iagi-net%40iagi.or.id/ IAGI-net Archive 2: http://groups.yahoo.com/group/iagi Komisi Sedimentologi (FOSI) : F. Hasan Sidi([EMAIL PROTECTED])-http://fosi.iagi.or.id Komisi SDM/Pendidikan : Edy Sunardi([EMAIL PROTECTED]) Komisi Karst : Hanang Samodra([EMAIL PROTECTED]) Komisi Sertifikasi : M. Suryowibowo([EMAIL PROTECTED]) Komisi OTODA : Ridwan Djamaluddin([EMAIL PROTECTED] atau [EMAIL PROTECTED]), Arif Zardi Dahlius([EMAIL PROTECTED]) Komisi Database Geologi : Aria A. 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