What's the meaning of Tags generated by stampy

Question

[Cherry]

Hello everyone!

I am confused with the tags in sam files generated by stampy.

I have checked the documents of stampy and it said that:

Stampy computes posteriors and likelihoods both for pairs of reads, and for reads considered by themselves. The following table summarizes the SAM tags for these statistics for paired reads:

      Read                     | Posterior   | Likelihood |
      -------------------------+-------------+------------+
      Pair                     | MAPQ column | PQ:i:      |
      This read as single read | SM:i:       | UQ:i:      |
      Mate as single read      | MQ:i:       | XQ:i:      |

and I got the results after mapping with stampy:

SRR******    99      chr1 40831563        99      100M    =       40831976        513     CTCCTTAGGTGTTAATTCCTTCCGGAGATAATCTATTATCTTTCCCTTCAAAAAAAAAAAAGATTGAGAGGTTTAAATCTCCCAATCCAATATTCGAGAG     @@@=DFDDHFHHHIIEIIIEHIIIH8E@FHEHAFFHEHIEHIIGGGGGHIIGAFHIIFCB688@CC>@AC@C<:?4@A:3>CCCCCC?(:>>ACD?AC?B     PQ:i:18 SM:i:58 UQ:i:0  MQ:i:96 XQ:i:0  NM:i:0
SRR******    147     chr1 40831976        99      100M    =       40831563        -513    AGCAAAATCCTGTACTGCCGCGCCACCTCTGTATCGGTGATTAGGATATCATCACCTAACAGGGCGTAATCAGCAAAAGGGGTAGTACTCGTTGGGTATG     ##@B@>:BBEBBB@B<BB@B??=?@@@6@@BB8:DIIEFGFCGEF@<<B8.88*EF@FDIGFFFEACIEIIFFDFIFFFEFFFIFGA@+A1FD?DDD@@@     PQ:i:18 SM:i:96 UQ:i:0  MQ:i:58 XQ:i:0  NM:i:0
SRR******    99      chr1 25433105        18      100M    =       25433519        514     AGTTAAGTAGTTCTATGCAACAGCAAAGTATAAAATAATTCTGGCAGCGTGGGCAAGACGTTGTATCACCACTTAGGCTCATAGTAGTGGTTGTCGGTTA  @@BDFFFDHHHGHJJIIIIJJJIIJJJHFHIJJJJJJJJJJJJJJIIGIDHIIGIIHJJIHEGCDGHEHHHHFFFFFDDEEEEDDDCFDD@?DDDDDB9B     PQ:i:512        SM:i:0  UQ:i:346        MQ:i:96  XQ:i:196        NM:i:9

My questions are

1. Which col is mapping quality? Is it col 5 or col PQ:i?
2. If I want to call SNPs next,I usually used XT tag to filter reads first after mapping with bwa, but how should I do now? Is it right to filter the reads with the mapping quality higher than 30 and properly paired at the same time using SAMtools?

If I have any misunderstanding, please tell me. Thanks a lot!