1

I am very new to QGIS and doing a project on Glacial Lake Outburst & Flooding (GLOF).

I am trying to find the area of a lake using images from Google Earth 7.0.3.8542. I am using the WGS84 CRS from my Google Earth image for the area calculation.

I am getting some messy area values like 0.000103145e-5. I can't even guess in what unit it's being displayed.

I have an official Local Coordinate System for our country in .prg format but I can't find it in the list of available CRSs in QGIS 2.0 or how to load it as a custom CRS.

How can I load the local CRS in my computer into QGIS?.

The contents of the .prg file are:

PROJCS["DRUKREF 03 local grid GASA",GEOGCS["GCS_DRUKREF03",DATUM["D_DRUKREF03",SPHEROID["GRS_1980",6378137.0,2‌​98.257222101]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],PROJECT‌​ION["Transverse_Mercator"],PARAMETER["False_Easting",250000.0],PARAMETER["False_N‌​orthing",-2500000.0],PARAMETER["Central_Meridian",90.0333333333333],PARAMETER["Sc‌​ale_Factor",1.0],PARAMETER["Latitude_Of_Origin",0.0],UNIT["Meter",1.0]]

10
  • 1
    What is your specific question?
    – Aaron
    Jun 21, 2014 at 19:44
  • First : How to calculate actual correct area in square meters or square kilometers using Google Earth images. Second : How to load a local Coordinate System for my country in QGIS
    – Smalbro93
    Jun 21, 2014 at 20:17
  • 1
    The answer to your second question solves your first question. Area calculations/tools will work with the units of the CRS (projection). As mentioned in my answer, your image's CRS uses degrees for units. I'm assuming your local CRS will use meters. Once you either convert (reproject) the image to or reproject it on the fly to your local CRS, the numbers you 'get' should be what you're looking for (or convertable to, ie meters to kilometers). Note your QGIS project has a projection which may or may not match all of the layers in that project - hence the 'on-the-fly' setting.
    – Chris W
    Jun 21, 2014 at 20:57
  • Thank you for answering. You are right, my local CRS uses meters. I tried this approach but I come to a dead end when I go to Project Properties check the Enable 'on the fly' CRS transformation then when I need to select my CRS I can't find my local CRS listed nor do i find a way to browse and load it into QGIS 2.0 that I am using.
    – Smalbro93
    Jun 21, 2014 at 21:04
  • You can add any CRS using Settings -> Custom CRS. Alternatively, you can take the UTM zone of your part of the world. Wikipedia has an overview map of all UTM zones, and QGIS has them all.
    – AndreJ
    Jun 22, 2014 at 10:41

3 Answers 3

2

Google Earth uses Simple Cylindrical projection with a WGS84 datum (source). It uses degrees of lat/lon for units, which are not terribly good for area calculations (nor is that projection really).

To use your official local CRS, you can do one of a few things. If it already exists in the projection list, you can use "Enable 'on the fly' CRS transformation" or export your image to a new file at which time you can set a different CRS than the one it's in. If your official local CRS is not already defined in the list, you need to load a custom CRS.

You can refer to the QGIS 2.0 Projection Help File for specific instructions on both methods. Note that Andre's answer gives you the specific string to paste into the CRS definition box based on the information supplied in your comment (basically translating the .prg file to PROJ.4 format), but his help file link currently leads to the 2.2 help file. There are very slight differences in the help, but the process should be the same in both versions.

There are also a number of similar/related questions already here on GIS.SE, including How to enable projection transformation in QGIS which indicates that on-the-fly raster projection has only been available since 1.7.

0
1

The local CRS you are looking for is EPSG:5295, DRUKREF Gasa. It was already implemented in the Windows version of QGIS 2.0.

If your copy does not have it, I suggest to update to QGIS 2.2, or wait a few days for QGIS 2.4

Alternatively, you can create a custom CRS with these parameters:

+proj=tmerc +lat_0=0 +lon_0=90.03333333333333 +k=1 +x_0=250000 +y_0=-2500000 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs

The necessary steps are explained in the QGIS docs:

http://docs.qgis.org/2.2/en/docs/user_manual/working_with_projections/working_with_projections.html

0

wgs84 uses lat lon as units,you are calculating area in this units, which is not useful. use an equal area-projection with units like meters or kilometers

1
  • Sorry but could you be a little more specific with "Use an equal area-projection with units like meters or kilometers" How to create an equal area projection for wgs84 CRS files. Sorry if I am not being clear.
    – Smalbro93
    Jun 21, 2014 at 20:11

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.