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I work on QGis 2.0.1 - Dufour on Macintosh OS X Mavericks.

I have UTM coordinates (XXXX and YYYYY) in a table (with one column of X and an other one of Y). I need this coordinates in Decimal Degree. Could you tell me the detailed protocol to follow please ?

  • Welcome to gis.stackexchange. Please note that a good question is generally supposed to show some basic research effort. Your question may have received downvotes because it seems that some of the steps are rather basic and the question could be more focused on a specific problem, e.g. do you have trouble loading the data? reprojecting the data? exporting the results? – underdark Mar 28 '14 at 17:44
  • Actly I've nevr been usin Qgis befor this urgent need. Tks tuto-vid &forums, but they don't answer precisely my quest°. Ifinally imported datas &made it editable,then opened the "field calculator" &tried to select the X&Y columns and wanted Qgis to convert them,from UTM to DD but couldnt change the coord units. It's all Icould do... Didnt try to reproject datas, cause Im already usin WGS84, and btw Idon't undstd why people answer "change RCS" while trying to convert the unit of coord dots ? Is UTM both a RCS AND a coord unit ? Thk you for your help, Hope Ishowed enough research efforts... – LauraP. Mar 28 '14 at 20:53
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Old question, but if you haven't figured it out by now UTM is not a coordinate unit. UTM is a projected coordinate system (aka CRS, what I think you mean in your comment) whose units are meters. WGS84 is properly a datum, but also refers to a geographic coordinate system whose units are degrees. Projected is flat, geographic is 'round'. It just so happens that the UTM projection system also uses WGS84 as the datum. This makes things a bit easier to convert coordinates between, as since they're on the same datum no transformation is required.

The suggestions to reproject the data are because (at least in ArcGIS, probably in QGIS as well) if you field calculate an x or y coordinate it does so based on the current projection. So if your points are in UTM, and you reproject them to WGS84, and then field calculate the x and y coordinates, you will get the decimal lat/long coordinates you seek. This method would be easiest because it's all set up in the software already (and is essentially the proper way to do the conversion you're talking about). The alternative is to create your own manual formula to do the calculation and perform it on the coordinate attributes (which, btw, aren't actually tied to the geometry - changing those values would not affect the point locations or their projection in any way).

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Googling this returns many informative pages. Here's one: http://www.uwgb.edu/dutchs/usefuldata/utmformulas.htm

  • Hy, I already googled this. The point is I'm trying to do it on Qgis (and this website explains how to calculate it by manual formulas). Anyway I could ask the same question for MapInfo as I will soon search on this one. What I'm looking for is an explanation step by step how to get the same table that I have (in UTM), in Decimal Degree... Do you know how ? – LauraP. Mar 27 '14 at 15:08
  • field calculator: qgis.org/en/docs/user_manual/working_with_vector/… – mikeLdub Mar 27 '14 at 15:47
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I create two functions (Datum WGS84)

http://gis.uazuay.edu.ec/ide/index.php?page=sql

We're looking for long answers that provide some explanation and context. Don't just give a one-line answer; explain why your answer is right, ideally with citations. Answers that don't include explanations may be removed.

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    This could be considered a link-only answer, which is generally discouraged on GIS.SE. If the link breaks, the answer becomes useless. It would improve the quality of the answer if you could edit it to describe that the two functions are SQL statements and that one is for converting in one direction and the other to do the reverse. It would greatly improve the quality of the answer if you were to include those two statements in the body of your answer. – Chris W Mar 3 '15 at 23:36

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