[GIS] Coordinate System Mismatch in QGIS

coordinate systemnad83qgiswgs84

I understand that coordinate system mismatches between layers is a relatively common problem. Nevertheless, exploring other posts hasn't helped me understand my issue. In short, I have a DEM raster and a KML route, and they should overlap based on geography alone, but they do not.

DEM File

I have downloaded a Vermont statewide DEM raster (the top option, "ALL_Elevation…") from this link: http://vcgi.vermont.gov/opendata. The data is in the NAD83 coordinate system per the included metadata text file.

KML File

I also have a KML file of a hike in Vermont in the WGS84 coordinate system. I'd post it here, but unfortunately, I cannot find a method of uploading the file with this post! It was generated using a GPS watch, and it happily imports into Google Earth without an issue. (I can send along the route if it may help troubleshoot this issue. Please just let me know.)

Attempts at a Solution

I've tried to first add the DEM file, set the project coordinate reference system (CRS) to NAD83 to match the DEM, enable "on the fly" CRS transformation, and then add the KML file. This did not work.

The opposite order didn't work either (i.e., first add the KML file, set the CRS to WGS84, enable "on the fly" CRS transformation, then add the DEM).

I've also attempted to add just the KML file to a new project and "reproject" the file using Save As and saving the file with a new name and using the NAD83 CRS. Adding the new and "reprojected" layer didn't overlap the DEM either.

Lastly, I've read in another post that data using a WGS84 CRS will have negative easting values. I would tack on the negative sign to all easting values just to try this, but I'm uncertain how to access the raw KML data.

Software Info

I'm running OS X 10.11.4 with QGIS 2.4.0 Chugiak.

UPDATE

Although I still haven't figured out what is going wrongly here, I do think that the issue is related to QGIS being unable to identify all the pertinent CRS information from the DEM layer. I think that a solution may involve defining a custom CRS using the "right" Proj4 code that matches my DEM. Proj4 codes can be found here: http://www.spatialreference.org. I've tried a couple different custom CRS ideas, but nothing works yet…

Best Answer

I've solved my issue. Long story short, I was importing the wrong elevation model file. See, the elevation data with which I'm working is an ArcGIS grid, so several files work together to provide a meaningful elevation model in QGIS. (Forgive the triviality here, but this is new to me. In my case, I was importing dem_10.ovr, which appears coherent when alone in QGIS, but the "correct" option was hdr.adf, which I think is the header file or something? Recall that I included a link to the Vermont state data I used in my question above.)

The main takeaway is that even if a file in a certain elevation model directory "appears" to be correct upon importation, it still may not be the correct file to use, because it may not connect to or contain all the pertinent information QGIS needs to re-project the data. If re-projecting the data doesn't seem to look right, one possible issue (if you're a newb like me) is that you're importing the incorrect elevation model file!

Related Solutions

[GIS] How to see the coordinate transformation parameters in QGIS

You can see the proj string used by QGIS to transform from and to WGS84 when you look at

Settings->Project settings, CRS tab, and search for the EPSG code or name of a CRS. All CRS are referenced by their EPSG code, or user defined CRS.

Uusally, all EPSG-codes are bundled with one certain transformation to WGS84 which was decided to be most appropriate. American NAD27 CRS are the only ones which use a ntv2 grid by default instead of three- or seven-parameter Helmert transformation.

If you are unhappy with the accuracy of a defined transformation, you can define a custom CRS with different +towgs84 parameters.

EDIT

For your Lisbon datum example, which is a projection of EPSG:4207, the EPSG database offers several transformation parameters which can be found in http://svn.osgeo.org/gdal/trunk/gdal/data/datum_shift.csv:

214,1656,4207,4326,"Parameter values from Lisbon to ETRS89 (1) (code 1655). Assumes ETRS89 and WGS 84 can be considered the same to within the accuracy of the transformation. Replaced by Lisbon to WGS 84 (4) (code 1988).",For applications to an accuracy of 3 metres.,1294,36.96,42.15,-9.55,-6.19,1,0,9606, -280.9,-89.8,130.2,-1.721,0.355,-0.371,-5.92,0

215,1944,4207,4326,"Parameter values from Lisbon to ETRS89 (2) (code 1790). Assumes ETRS89 and WGS 84 can be considered the same to within the accuracy of the transformation.",For applications to an accuracy of 2 metres.,1294,36.96,42.15,-9.55,-6.19,1,1,9606, -282.1,-72.2,120,-1.592,0.145,-0.89,-4.46,0

216,1984,4207,4326,,For low resolution applications.,1294,36.96,42.15,-9.55,-6.19,1,0,9603, -304.046,-60.576,103.64,,,,,1

217,1988,4207,4326,,For medium resolution applications.,1294,36.96,42.15,-9.55,-6.19,1,0,9607, -288.885,-91.744,126.244,1.691,-0.41,0.211,-4.598,0

So you see, that the low resolution transformation with three parameters is used in your example. It is stored in http://svn.osgeo.org/gdal/trunk/gdal/data/gcs.csv under code 4207 and used by Qgis and other GIS software depending on proj.4.

http://www.epsg-registry.org/ gives an accuracy of 5 metres for EPSG:1984 (note that transformations have their own code numbers). These values were first published by NIMA when WGS84 was introduced. But its not what the local surveying authority recommends. You can also have a look at http://www.fc.up.pt/pessoas/jagoncal/coordenadas/index_en.htm

If you want it more accurate, try the ntv2 grid from http://www.igeo.pt/produtos/geodesia/Grelhas_NTv2.htm

[GIS] On the fly transformation From WGS84 to NAD27 with proj4 code

The layers are in WGS84, and the project CRS is NAD27 UTM 19 North.

I am not 100% sure this will work, but the convention in PROJ.4 for 3 and 7 parameter transformations is to use

+towgs84=-3,142,183,0,0,0,0

In the project properties, CRS tab, PROJ.4 text box, enter this definition:

+proj=utm +zone=19 +datum=NAD27 +ellps=clrk66 +towgs84=-3,142,183,0,0,0,0

If it doesn't appear to be working, try dropping the +datum=NAD27 option.

Best Answer

Related Solutions

[GIS] How to see the coordinate transformation parameters in QGIS

[GIS] On the fly transformation From WGS84 to NAD27 with proj4 code

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