Pauline, It depends on the date of the Paraguayan map. Note that the predecessor of SAD69 is/was the Provisional South American Datum of 1956 (PSAD56) with its origin point at La Canoa, Venezuela. Northern South America remained on the PSAD56 until a couple years ago, when the entire continent switched to SIRGAS. However, ALL of the existing maps remain on the old classical datums. Only NEW compilations are going to be referenced to SIRGAS. Note that most countries in South America now publish transformation parameters from their classical datums to SIRGAS. Also note that you have to look at the scale of the map and its publication date as well as its edition versus original compilation date. Many geological maps are small-scale; e.g., 1:250,000, 1:500,000, etc. and really the datum is immaterial since positional accuracy of 100-200 meters is the best you could possibly scale off the map even if you do a fine digital scan. Datums become important at around 1:100,000 scale, they get very important at 1:50,000 scale for military artillery purposes, and at 1:25,000 and larger are of great importance in surveying and point positioning. At 1:1,000 scale, you will need dual-frequency geodetic receivers to tell the differences in local native datum transformations. Read my stuff, in particular: "The Basics of Datums" at www.ASPRS.org/resources/GRIDS - it's the April, 2000 column. ------------------------------------- Thanks for the info. This is great. So, if I have a Paraguay geology map (pre-1980) with lat-long coordinates around the perimeter and no other information, do you think I can assume the reference ellipsoid is SAD69?? Cheers Pauline Honarvar, M.Sc., P.Geo. Spatial Data Management ----- Original Message ----- From: "Clifford J Mugnier" <[EMAIL PROTECTED]> To: "P. Honarvar" <[EMAIL PROTECTED]> Cc: <[email protected]> Sent: Wednesday, August 17, 2005 12:28 AM Subject: Re: MI-L Chua Astro parameters Pauline, "Chua Datum" is not a projection. It is as listed, a datum origin. The Chua Origin point is in Brazil where the astronomic Latitude is: 19º 45' 41.6527" N, and Longitude is: 48º 06' 04.0639" West of Greenwich. The defining azimuth of the South American Datum of 1969 at station Chua is: 271º 30' 04.05" to station Uberaba. The orthometric height (elevation) is 763.28 meters above local mean sea level. The reference ellipsoid of SAD69 is the International 1909 where a = 6,378,388 meters and 1/f = 298.25. Note that station Chua is in Brazil, not in Paraguay, even though they use it for a reference system - it used to be the "standard for the southern half of the continent. (The new system is called "SIRGAS.") Chua is not a datum and certainly not a projection. SAD69 is a datum, and the "projection" associated with that datum is the Gauss-Krueger Transverse Mercator with UTM for its standard parameters as a Grid system. Check out my column on neighboring Uruguay, November 2002 in "Photogrammetric Engineering and Remote Sensing" available from www.ASPRS.org/resources/GRIDS - there's a detail or two in there that might help you with your MapInfo problems. Clifford J. Mugnier Chief of Geodesy and Associate Director, CENTER FOR GEOINFORMATICS Department of Civil Engineering CEBA 3223A LOUISIANA STATE UNIVERSITY Baton Rouge, LA 70803 Voice and Facsimile: (225) 578-8536 [Academic] Voice and Facsimile: (225) 578-4474 [Research] ====================================================== http://www.asprs.org/resources/GRIDS/ http://www.cee.lsu.edu/facultyStaff/mugnier/index.html ====================================================== Dear List I have a topo map of part of Paraguay which is in UTM, zone 21, Chua Astro datum. I can't find this on the MapInfo projections list (v. 7.8). Where would I find the parameters needed to add this projection to the MapInfo projections file? TIA Pauline --------------------------------------------------------------------- List hosting provided by Directions Magazine | www.directionsmag.com | To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED] Message number: 17514 --------------------------------------------------------------------- List hosting provided by Directions Magazine | www.directionsmag.com | To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED] Message number: 17521
