I am having such pandas values as follows:

df1 = pd.DataFrame({
    'Lat':('-10.1','-10.2','-10.3','-10.4','-10.5','-10.6'),
    'Lon':('90.000','90.050','90.100','90.150','90.200','90.250'),
    'Position':('-9999.999','-9999.999','-9999.999','-9999.999','-9999.999','-9999.999')})
df1 = df1.set_index('Lat','Lon')

df2 = pd.DataFrame({
    'Lat':('-10.2','-10.5'),
    'Lon':('90.050','90.200'),
    'Position':('50','100')
})
df2 = df2.set_index('Lat','Lon')

output=df1.combine_first(df2).reset_index()

and the answers as follows:

     Lat     Lon   Position
0  -10.1  90.000  -9999.999
1  -10.2  90.050  -9999.999
2  -10.3  90.100  -9999.999
3  -10.4  90.150  -9999.999
4  -10.5  90.200  -9999.999
5  -10.6  90.250  -9999.999

However, I am expecting the output as follows:

     Lat     Lon   Position
0  -10.1  90.000  -9999.999
1  -10.2  90.050  50.0
2  -10.3  90.100  -9999.999
3  -10.4  90.150  -9999.999
4  -10.5  90.200  100.0
5  -10.6  90.250  -9999.999

anynone got an ideas, please help. Thanks in advance.