[GIS] Exporting geographic coordinates in QGis as projected (flat) coordinates

coordinate systemkmlqgis

I've created some polygons with Google Earth which I'd like to convert to my flat map's coordinate system.

I've loaded a few shapefiles into QGis of a county's roads, parcels, etc. The coordinate system for these points is projected using the "NAD83(HARN) / South Carolina (ft)" or "EPSG:3361" standard.

Next I load a kml file into QGis created using Google Earth Pro which is composed of a Polygon of geographic (lon/lat) points.

QGis is clever and has perfectly overlaid the geographic-points polygon directly where it should be on the flat/projected-points polygon. I can hover my mouse around and see the projected coordinate points where my polygon's exterior ring resides.

I want to export the points of my geographic-polygon into the projected coordinate system that the rest of my qgis map is based on. This is because all of my county data shapefiles are projected, that and PostGIS doesn't seem to allow me to do spatial joins of two geographic polygons…

Anyway, I right click on the imported vector layer (Google Earth-gen'd polygon) and click 'Save As'. (I've already enabled 'On the Fly CRS Transformation')

The plan is to export the polygon into projected coordinates, so I choose 'Selected CRS' and locate my proj4 code which is for SRID:EPSG-3361.

Exporting this does not accomplish my goal. The exported kml still contains geographic coordinates. QGis seems very capable of doing what I want to do, its overlaid two layers from entirely different coordinate systems. How might I go about transforming this clearly-understood (by qgis) polygon into the projected coordinate system upon which it apparently lies?

UPDATE 1:
– I created a second layer whose chosen CRS is the projected one, redrew a similar polygon on top of the imported one, and tried to export it, using, again, the projected coordinate system. IT ALSO got exported into geographic coordinates!

UPDATE 2:
I exported a shapefile of the county this time and IT TOO got exported into geographic coordinates!!! This tells a lot about the nature of the problem. Everything is being exported into geographic coordinates. Could it just be the fact that I'm using kml?

Best Answer

Kml has always degrees as units, so you will have no luck with that format.

Exporting as shapefile should work. Note that you have to choose the CRS again (QGIS does not remember it from the last export).

Can you add the shapefile to the canvas, and look up the extent under properties, metadata tab? Furthermore, you can open the .prj file that was created along with the shapefile. It should contain the projection data for NAD83(HARN) / South Carolina (ft).

Related Solutions

[GIS] How to read geographic coordinates when the Shapefile has a projected spatial reference

For you, what is the difference between geographic coordinates and projected coordinates ?

From ESRI GeoNet: Projected Coordinate System vs. Geographic Coordinate System:

A Geographic Coordinate Systems is defined by a 3-D surface and is measured in latitude and longitude.
A Projected Coordinate systems refers to data that is defined by a flat 2-D surface and can be measured in units of meters and feet.
When displaying data that's using a geographic coordinate system, ArcMap uses a 'pseudo-Plate Carree' projection. Basically, it just treat the coordinate values as if they're linear and just display the data.

Therefore if your coordinates are lat & lon variables, you use a Geographic Coordinate System (sensu ESRI)

1) With osgeo.ogr and a shapefile with WGS84 projection (Geographic Coordinate System), the crs is found by:

from osgeo import ogr
infile = ogr.Open("aWGS84.shp")
layer = infile.GetLayer()
# crs
spatialRef = layer.GetSpatialRef()
spatialRef.ExportToWkt()
'GEOGCS["GCS_WGS_1984",DATUM["WGS_1984",SPHEROID["WGS_84",6378137,298.257223563]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]]'

You can also use spatialRef.ExportToPrettyWkt(),spatialRef.ExportToProj4(),spatialRef.ExportToUSGS() and many other formats.

First feature of the shapefile

feature = layer.GetFeature(0)
feature.ExportToJson()
'{"geometry": {"type": "Polygon", "coordinates": [[[5.826429692987699, 50.63093772365397], [5.830982302422351, 50.62928355089628], [5.826407932565759, 50.6268451632606], [5.82334832264974, 50.62941225500874], [5.826429692987699, 50.63093772365397]]]}, "type": "Feature", "properties": {"id": null}, "id": 0}'

And the point coordinates of the Polygon (WGS84 in degrees)

import json
json.loads(feature.ExportToJson())['geometry']['coordinates']
[[[5.826429692987699, 50.63093772365397], [5.830982302422351, 50.62928355089628], [5.826407932565759, 50.6268451632606], [5.82334832264974, 50.62941225500874], [5.826429692987699, 50.63093772365397]]]

2) With the same shapefile in a projected coordinate system (in meters)

infile = ogr.Open("aproj.shp")
layer = infile.GetLayer()
feature = layer.GetFeature(0)
json.loads(feature.ExportToJson())['geometry']['coordinates']
[[[253122.92278611325, 147711.88454888575], [253448.57840302447, 147534.25421241205], [253130.3240500753, 147256.7068115687], [252908.28612947493, 147537.9548443528], [253122.92278611325, 147711.88454888575]]]

3) For the conversion between the two geometries, I need to reproject the shapefile (degrees to meters) with osgeo.osr or Pyproj (there are many examples in GIS SE).

4) Instead of using osgeo.ogr why don't use the more "Pythonic" Fiona (another Python wrapper of the OGR library)

import fiona
file = fiona.open("aWGS84.shp")
spatialRef = file.crs
spatialRef
{'init': u'epsg:4326'}

Fiona use Python dictionaries for all (GeoJson format), therefore the first feature is obtained by

file.next()
{'geometry': {'type': 'Polygon', 'coordinates': [[(5.826429692987699, 50.63093772365397), (5.830982302422351, 50.62928355089628), (5.826407932565759, 50.6268451632606), (5.82334832264974, 50.62941225500874), (5.826429692987699, 50.63093772365397)]]}, 'type': 'Feature', 'id': '0', 'properties': OrderedDict([(u'id', None)])}

[GIS] Exporting polygon coordinates from R to Google Earth

Yes, definitely possible.

Assuming your [LongLat.csv] has longitude (lon), latitude (lat), and elevation (z) columns.

MyData <- format(read.csv("LongLat.csv", header = FALSE), nsmall = 5)
colnames(MyData) <- c("lon", "lat", "z")

MyData$lon_lat_z <- paste0(MyData$lon, ", ", MyData$lat, ", ", MyData$z)
paste(unname(unlist(MyData$lon_lat_z)), collapse = " ")

Then you can copy the output from your R console, and paste it into your [coordinates] tags.

Best Answer

Related Solutions

[GIS] How to read geographic coordinates when the Shapefile has a projected spatial reference

[GIS] Exporting polygon coordinates from R to Google Earth

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