I am producing DEMs and various maps for own book project; just learned to use GIS software for this project. I am using QGIS (2.14.21) and SRTM data (CRS:WGS84 EPSG:4326). I want square maps that extend a specific distance in four main directions (E,W,N,S) from the center (38.504421, 43.331467 or 38° 30' 15.9156'' N 43° 19' 53.2812'' E).This was my procedure: in Bing Maps, measured 100km to west from center & 100km to north from there to find coordinate1. did same thing to east&south to find coordinate2. checked coordinates&distances in Google earth. then used the coordinates to clip the raster image in QGIS. did this for 100km, 200km and 400km.
Now I have "rectangular" maps instead of "square" maps. I measured 400km radius images in QGIS, they are correct in N & S, but QGIS shows a little above 500km in E & W. Measurements are same for DEM and raster images. In G.Earth when I measure 400km from my center point to east there is (for ex.) a mountain; that "same mountain" is in correct position in my clipped images in QGIS (correct, according to my procedure, 400km radius), but when I measure the distance from center to that mountain in QGIS, I get 500km. This measurement difference occurs only in east and west.
I searched for an explanation and concluded that this shape difference is the result of map projection. But I am biology teacher in profession and I have very little knowledge about GIS, cartography, etc. But I need to know whether "map projection" is the answer or I must correct all of my maps. Because, in a chapter of the book I will argue the effect of map projection (the distortion) and I will use the shape of my maps as an example. Also I must give the distance information correctly in all chapters to explain the limits of the area in the maps. I could only use QGIS measurements to clip original rasters; I didn't do this, because I wanted to pinpoint a center location in satellite photos and G.Earth seems to measure distances more accurately (like length of a street).
Is rectangular maps OK? Are all these the result of "map projection"? If I must correct the maps, how can I do it?