Inconsistent results from DTEDFrame.interpElevationAt()

[carpenlc]

The method DTEDFrame.interpElevationAt() is producing unexpected values for the output elevation value. As an example, using a sample DTED2 elevation file and point lat=-14.123 (index=3157.1995) and lon=-68.555 (index=1601.9989) the nearest 4 elevation posts in this dataset are:

ur = 1513
ul = 1522
ll = 1523
lr = 1515

Manually calculating a bilinear interpolation shows that the output elevation should be very close to the lower-right elevation post which is 1515 meters, but the resulting value is near the upper-right post (i.e. 1513). The code is not utilizing the lower elevation posts to generate the output interpolated value.