Distorted KML Ground Overlay in QGIS

Question

Still learning my way around QGIS and ran into an odd issue this evening. I'm using a program called Radio Mobile Deluxe to generate RF models. It exports images as KML ground overlays. I then bring the KML into QGIS, then georeference the raster.

However, this evening, I noticed that things didn't look quite right. I should be looking at perfectly square objects (8000x8000 meters and a 800x800 pixel raster).

The overlay imports perfectly into Earth, but when I bring it into QGIS, it's measuring out 10km wide. However, when I compare visual features in the imagery basemap and roads, everything looks dead on. I'm really not sure what I'm looking at here, if it's a problem, or how to fix it.

I'm using QGIS 2.18.4, the project CRS is 4326. Not quite sure what other information I should provide.

Here's a screenshot, showing a very square object in Earth and a not-so-square object in QGIS, along with the N/S/E/W coordinates generated by Radio Mobile.

Here's a sample KML file I created in Earth and loaded into QGIS:

<?xml version="1.0" encoding="UTF-8"?>
<kml xmlns="http://www.opengis.net/kml/2.2" xmlns:gx="http://www.google.com/kml/ext/2.2" xmlns:kml="http://www.opengis.net/kml/2.2" xmlns:atom="http://www.w3.org/2005/Atom">
<Document>
    <name>foo.kml</name>
    <Style id="s_ylw-pushpin_hl">
        <IconStyle>
            <scale>1.3</scale>
            <Icon>
                <href>http://maps.google.com/mapfiles/kml/pushpin/ylw-pushpin.png</href>
            </Icon>
            <hotSpot x="20" y="2" xunits="pixels" yunits="pixels"/>
        </IconStyle>
        <LineStyle>
            <color>ff0000ff</color>
            <width>3.5</width>
        </LineStyle>
        <PolyStyle>
            <fill>0</fill>
        </PolyStyle>
    </Style>
    <StyleMap id="m_ylw-pushpin">
        <Pair>
            <key>normal</key>
            <styleUrl>#s_ylw-pushpin</styleUrl>
        </Pair>
        <Pair>
            <key>highlight</key>
            <styleUrl>#s_ylw-pushpin_hl</styleUrl>
        </Pair>
    </StyleMap>
    <Style id="s_ylw-pushpin">
        <IconStyle>
            <scale>1.1</scale>
            <Icon>
                <href>http://maps.google.com/mapfiles/kml/pushpin/ylw-pushpin.png</href>
            </Icon>
            <hotSpot x="20" y="2" xunits="pixels" yunits="pixels"/>
        </IconStyle>
        <LineStyle>
            <color>ff0000ff</color>
            <width>3.5</width>
        </LineStyle>
        <PolyStyle>
            <fill>0</fill>
        </PolyStyle>
    </Style>
    <Placemark>
        <name>Untitled Polygon</name>
        <styleUrl>#m_ylw-pushpin</styleUrl>
        <Polygon>
            <tessellate>1</tessellate>
            <outerBoundaryIs>
                <LinearRing>
                    <coordinates>
                        -79.22887,38.08849,0 -79.13737,38.08849,0 -79.13737,38.08849,0 -79.13737,38.16048,0 -79.22887,38.16048,0 -79.22887,38.16048,0 -79.22887,38.16048,0 -79.22887,38.08849,0 
                    </coordinates>
                </LinearRing>
            </outerBoundaryIs>
        </Polygon>
    </Placemark>
</Document>
</kml>

EDIT: Here's what the raster looks like (after converting to polygon and removing the background).

When I spot-check the overlay down to the pixel, it's identical between QGIS and Earth. Accurate enough for modeling RF signals, which tend to get pretty fuzzy in the real world.

Answer 1

It seems like you're having the expected behaviour.

Google Earth does not distort your image while QGIS is ! Due to it's 3D Globe, Google Earth can be equal of angle AND area (disclaimer: unless you have a 3d screen, projection onto your 2d screen will of course distort your angle too, but that is not noticeable on that scale).

As coordinates on the globe are always 3D, QGIS needs to project them somehow onto the map on your 2D screen. Thus, the image will always be distorted. The projection commonly used for showing WGS84 (EPSG:4326) coordinates is Plate Carree (longitude=x, latitude=y). If you're looking on the whole Globe using this projection, the further you go north or south the more distorted it will be.

(Picture: Eric Gaba – Wikimedia Commons)

This is why a lot of CRS exist, most of them suitable only for one specific region of the earth where it shows only very little distortion.

What you can do:

Change the projection of your map in QGIS to the UTM Zone of your region (or maybe play around using Mercator (EPSG:3857) and you will have your almost perfect square.

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