The EXIF altidudes in the images (DJI Phantom 3 FC300X camera) have positive values; the values imported into Photoscan (Pro Windows 1.2.6 build 2843, 64 bit) are negative. Same numbers, but with a minus sign.
This is annoying, but the workaround is simple: export the camera locations, edit to remove the minus signs, and re-import...or use a script to scan the EXIF info in the images and generate the file to import, which may be preferable, because as others have pointed out, the actual values are off by some offset that could be removed in the same script.
Camera info after loading photos in PhotoScan:
# CoordinateSystem: GEOGCS["WGS 84",DATUM["World Geodetic System 1984",SPHEROID["WGS 84",6378137,298.257223563,AUTHORITY["EPSG","7030"]],TOWGS84[0,0,0,0,0,0,0],AUTHORITY["EPSG","6326"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.01745329251994328,AUTHORITY["EPSG","9102"]],AUTHORITY["EPSG","4326"]]
#Label,X/East,Y/North,Z/Altitude
DJI_0012.JPG,-70.486803,41.768299,-154.170000
DJI_0013.JPG,-70.486481,41.768133,-154.170000...
EXIF info as read by IrfanView (EXIF plugin):
Filename - DJI_0012.JPG
ImageDescription - DCIM\100MEDIA\DJI_0012.JPG
Make - DJI
Model - FC300X
Orientation - Top left
XResolution - 72
YResolution - 72
ResolutionUnit - Inch
Software - v01.30.5036
DateTime - 2017:01:09 12:48:27
...more info...
GPS information: -
GPSVersionID - 3.2.0.0
GPSLatitudeRef - N
GPSLatitude - 41 46 5.88 (41.768300)
GPSLongitudeRef - W
GPSLongitude - 70 29 12.49 (70.486803)
GPSAltitudeRef - Sea level reference
GPSAltitude - 154.17 m