Buffer Creation in Meters – How to Create Buffer in Meters

buffercsvqgisunits

Objective

Given a list of coordinates in longitude/latitude, stored as a .csv file, I want to create a buffer with distance of x km around those coordinates. (In case this is relevant, these coordinates are locations of the societies in the SCCS dataset. These societies scatter all around the world.)

Problem

I'm only able to create buffers with distance in radial degrees, but I want to do them in (kilo)meters. I'm aware of the following Q&A's, but following what's suggested therein doesn't seem to solve my problem:

What I have tried…

I start QGIS 2.4, and Add delimited text layer
choose my .csv file and the x/y fields, click OK
select WGS 84 as my CRS (also tried NAD83 but didn't make a difference), click OK
right-click on layer and select Save As..., in the dialog box, choose the following and click OK:
add sccs_meter.shp as layer to the current project
Select Vector > Geoprocessing Tools > Buffers > set buffer distance as 10

The result is a buffer with radius of approximately 1068km, which suggests that buffer distance is 10 degrees rather than 10 meters.

What did I do wrong here?

Best Answer

In step 4, you have to change the CRS from NAD83 to another projection that uses metres as units.

It depends on the extent of your data which one is best. Unfortunately, your data is located all over the world, so you could:

Create a custom CRS using aeqd (or tmerc) for each one, and draw just that one buffer with it. Practically, you only have to create the buffer once, and exchange the CRS information in the .prj and .qpj file. The coordinates of the buffer with respect to its center will always be the same.
Group the data according to the UTM zones, and use the UTM CRS of that zone for those points.
Similar to UTM, group your points into zones of latitude (e.g. every 10 degrees), and create custom Lambert conformal conical 2SP CRS for each group. This will be significantly faster than using all northern and southern UTM zones of the world.
Use pseudo mercator EPSG:3857 for all. The buffers will look like nice circles, but the real size will get smaller and distorted the more to the poles you come.

Related Solutions

Buffering in Meters/KM Using WGS84 Layers in QGIS

For this application, I would use an Azimuthal Equidistant projection centered in the middle of your source points. This projection has the nice feature of all radial distances around the center of the projection being accurate.

That particular projection is not part of QGIS standard projections. You can define your own using Settings/Custom CRS with the command string +proj=aeqd +lat_0=24.5 +lon_0=121.5, but unfortunately, custom projections can't be used by the fTools Reprojection tool. Instead, you can transform your dataset on the command line using the command

ogr2ogr points_reprojected.shp points.shp -t_srs "+proj=aeqd +lat_0=24.5 +lon_0=121.5"

Then you can do the buffering in QGIS using the points_reprojected.shp shapefile.

A 2000km buffer around a point in north Taiwan looks like a circle in an orthographic projection centered on the point:

... and squashed in WGS84:

[GIS] Need Projection Help to Buffer Lines in QGIS

Something most likely went wrong with your projection settings along the way, but it's a bit hard to figure out where exactly the error lies, so it might be easier to walk through the process from the start.

When you load the shapefile, you'll notice that you're asked to pick the coordinate reference system (CRS) your shapefile is in. Usually, this dialog won't pop up, because the correct coordinate system is stored in one of the files that make up the shapefile: the one ending in .prj. That one's missing with your dataset, however, hence the dialog. So we need to find out what CRS your data is in. Without this information, any projections we apply later on will be meaningless.
Googling for rs14fe02 (the base name of your shapefile) points to the NOAA metadata entry. The section "Spatial reference information" tells us that the data is in geographic coordinates (i.e. it uses latitude and longitude, not distance units), and that it uses the North American Datum of 1927 with the Clarke 1866 ellipsoid. Scrolling through the list of CRS's in the QGIS dialog, we find an entry called NAD27 in the "Geographic Coordinate Systems" section. The proj string is +proj=longlat +ellps=clrk66 +towgs84=-10,158,187,0,0,0,0, so that matches the metadata.
After choosing the NAD27 CRS and clicking OK, the shapefile is opened:

I've added a graticule of longitude and latitude: As you can see, they're perfectly square, which is what you'd expect from a geographic coordinate system displayed in QGIS, since it just uses an equirectangular projection to display the data. If you measure a distance using the line measure tool, it's in degrees, which again, is what you'd expect, but not what you want for buffering using linear units.
So we need to project the data. For buffering, we should use an equal area projection to avoid introducing errors into the analysis. A common one is the "US National Atlas Equal Area" (EPSG:2163), which is a Lambert azimuthal equal area projection. You can see what your data looks like in that projection by clicking on the globe in the lower right corner, activating "on the fly projection", and choosing EPSG:2163 from the list:
You might think that you could just create the buffer now, but you'd be disappointed if you did: The geoprocessing tools don't care what projection is selected for the project, they care what projection the data is in. We haven't changed that: The shapefile is still in NAD27 geographic coordinates, so running a buffer with a buffer distance of 3000 "units" would be interpreted as 3000°, not 3000 metres. Instead, we explicitly have to project the data first: Select the layer, and click "Layer / Save as...", choose a file name, and choose "Selected CRS" and "EPSG:2163". Tick "Add saved file to map" and click OK. After the file is saved, QGIS will load and display it.
Now you can run the buffer with the proper units: Open the "Vector / Geoprocessing / Buffer" dialog, choose the newly created layer, and enter the buffer distance in metres. Once it's done, the layer will be added to the canvas.

Here's a section from the north eastern US with a 5 km buffer:

Best Answer

Related Solutions

Buffering in Meters/KM Using WGS84 Layers in QGIS

[GIS] Need Projection Help to Buffer Lines in QGIS

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