Towed magnetometer records are examined from 70 cruises in the Indian Ocean,
covering the years 1960 to 1990. Geomagnetic secular variation was estimated
from readings at track crossings and compared to the secular variation
as recored at five geomagnetic observatories on land around the Indian
Ocean and repeat stations in Western Australia. Even when periods of high
kp are excluded, or when model corrections are made for short time variations
of the field, the correlations with observatories is only fair. Spatial
filtering the along track data does not improve the correlation.
There is some internal consistancy of the values
found in the marine areas, suggesting that secular variation there is different
in areal distribution from that found on land. Satellite (Magsat and Oertsted)
measurements were made nearly 20 years apart so they cannote be expected
to show shortterm changes of the secular vaariation found in magnetic observatory
records or short term track crossings.