I'm using QGIS 3.4.1, and I'm trying to print 61km x 61km area map of somewhere South Korea. Then I need to send this map with west / east latitude and north / south longitude info to other person, who need to do some work on that map. Here's what I did.
First, I set project CRS to EPSG 32652, WGS84 - UTM zone 52N, because most of South Korea is in the zone 52N.
Second, I load maps from Quick Map Service plugin, ESRI map from ArcGIS Map Server layer, Openlayers plugin. I also downloaded SRTM data using SRTM downloader, and also from CGIAR-CSI for double check.
Third, I found out that all of layers that I loaded or downloaded have CRS of EPSG 3857, WGS84 - Pseudo Mercator. I intuitively thought that they should have the same CRS with project CRS. So I changed CRS of all layers to project CRS, which is EPSG 32652, WGS84 - UTM zone 52N.
Then I found out something gone wrong seriously. Distance is distorted, grids, scale bar, and ruler show unusually shorter distance than it should be. In addition, I found that lat long coordinate shown from Lat Long tools plugin is really really off from real Lat and Long coordinate. For example, typical Lat Long position of Seoul is around 37.55 N, 127.03 E. However, Lat Long position from map after map CRS conversion shows around 8.45 N, -151.53 E. Moreover, all SRTM data layer moved to really strange position.
Just in case, I repeated the same procedure from QGIS 2.18 with on-the-fly CRS transformation option on. But the same symptom occur.
I think major problem here is the CRS, project CRS and layer CRS. I think my "I intuitively thought that they should have the same CRS with project CRS. " this part is wrong, but I can't understand.
Why should I need to use WGS 84 Pseudo-Mercator CRS for layers, while using WGS 84 UTM 52N CRS for project?
And I don't know what to tell my coworker, which CRS I used for this 61km x 61km area map.