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.