[GIS] Exporting polygon shapefile with lat/lon points

formatlatitude longitudeqgis

By copying rows of the attribute table of a polygon shapefile, I figured out how to extract the points of the given polygon but to my surprise, they are formatted backwards:

POLYGON((
-87.529571533203125 34.55934906005859375,
-87.52953338623046875 34.3049163818359375,
-87.93964385986328125 34.31328964233398438,
-88.17311859130859375 34.32147979736328125,
-88.1404571533203125 34.58187484741210938,
-87.95177459716796875 34.57530975341796875,
-87.52912139892578125 34.56753158569335938,
-87.529571533203125 34.55934906005859375))

Is there a way to export these points in the same way but formatted as lat/lon xx.xx, -xx.xx points?(only US counties are being processed).
If not and post-formatting is the only option, if there is anyone out there who has already done this…how?
If not I guess I will just have to write a formatting script but it will be a pain to execute on all 4703 counties.

Best Answer

Use MMQGIS plugin

It is available from the Manage and Install Plugins item, search for mmqgis

These tools will appear with their own menu item MMQGIS, use the Import / Export | Geometry Export to CSV.

Two files will be created. You want the "nodes"

Once you have this new file in a spreadsheet, move the columns around. Then the next steps depend on the import requirements of your program. Look at the structure of the nodes file created by MMQGIS and note how the shapeid column gives an index of the polygons.

Related Solutions

[GIS] Creating regularly spaced, defined # of points within polygon in QGIS

What QGIS is referring to in terms of the layer boundary is actually the envelope or bounding box of the geometry.

I've documented a solution which is probably the closest you will get to automating this is (short of filing a bug / feature request).

The process works based on the ratio between area of the two objects: the feature geometry and it's bounding box).

Check the following steps below:

Select the layer you want to work on.
Select the feature you wish to work on. Make sure nothing else is selected.

Run the following snippet within the Python Console.

layer = qgis.utils.iface.activeLayer()
features = layer.selectedFeatures()
feature = features[0]
geom = feature.geometry()
env_rect = geom.boundingBox()
env_geom = QgsGeometry.fromRect(env_rect)
env_geom.area() / geom.area()

Be sure to push enter as the last line will not automatically return.

Run the Regular Points tool again, but enter the number of points that you want inside the box multiplied by the output given by your script.

In my sample below, the result I received from the output was a value of ~2. I requested 20 dots to be output by the process where I actually only wanted 10. I was given 8 in return, which is fairly close. Depending on the actual shape you're using the results will vary.

If you were so inclined, you could automate (recursively) the Regular Points process modifying the variables until you arrived at your desired number.

Sample

[GIS] How to determine vector between two lat/lon points

What you're looking for is the initial bearing (or forward azimuth), which if followed in a straight line along a great-circle arc will take you from the start point to the end point.

Here is some simple JavaScript from this link:

var y = Math.sin(dLon) * Math.cos(lat2);
var x = Math.cos(lat1)*Math.sin(lat2) -
        Math.sin(lat1)*Math.cos(lat2)*Math.cos(dLon);
var brng = Math.atan2(y, x).toDeg();

The above link has a wealth of useful information beyond this for related calculations.

As your question states, this is the simplest method - since the Earth is not a true sphere this calculation will not be 100% accurate, but it is a close approximation.

Best Answer

Related Solutions

[GIS] Creating regularly spaced, defined # of points within polygon in QGIS

[GIS] How to determine vector between two lat/lon points

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