very early JTT prototype, created from https://github.com/dret/JTT/blob/master/GPX/test.gpx via https://github.com/dret/JTT/blob/master/GPX/GPX2JTT/GPX2JTT.xsl

test.json

{ "JTT" : [
   { "track" : {
     "title" : "Test Activity",
     "desc" : "Simple test activity walking around the block with a heart rate monitor, stopping once, and marking one lap. Recorded with Garmin Fenix 3, uploaded to Garmin Connect, and exported as GPX and TCX. Both exports are crippled because they do not contain all data fields of the activity.",
     "segments" : [
       { "data-fields" : ["latitude", "longitude", "elevation", "temperature", "HR" ] }, 
       [
         [ 47.407614681869745, 8.553115781396627, 451.79998779296875, "2015-11-13T12:57:24.000Z", 28.0, 76],
         [ 47.40762138739228, 8.553108656778932, 451.0, "2015-11-13T12:57:25.000Z", 28.0, 76],
         [ 47.407626835629344, 8.553094072267413, 450.0, "2015-11-13T12:57:26.000Z", 28.0, 76],
         [ 47.407668409869075, 8.552960716187954, 449.0, "2015-11-13T12:57:33.000Z", 28.0, 78],
         [ 47.40770772099495, 8.552867090329528, 448.79998779296875, "2015-11-13T12:57:39.000Z", 28.0, 75],
         [ 47.407753402367234, 8.5526111908257, 448.79998779296875, "2015-11-13T12:57:54.000Z", 27.0, 75],
         [ 47.40781215950847, 8.552352441474795, 448.20001220703125, "2015-11-13T12:58:04.000Z", 27.0, 73],
         [ 47.407849207520485, 8.552198382094502, 448.20001220703125, "2015-11-13T12:58:12.000Z", 27.0, 76],
         [ 47.40790771320462, 8.552007442340255, 448.0, "2015-11-13T12:58:23.000Z", 27.0, 75],
         [ 47.40796320140362, 8.551924461498857, 447.79998779296875, "2015-11-13T12:58:29.000Z", 27.0, 78],
         [ 47.4079969804734, 8.551919935271144, 447.6000061035156, "2015-11-13T12:58:32.000Z", 27.0, 78],
         [ 47.408049115911126, 8.551959414035082, 447.20001220703125, "2015-11-13T12:58:37.000Z", 27.0, 77],
         [ 47.408086750656366, 8.552005849778652, 446.79998779296875, "2015-11-13T12:58:41.000Z", 27.0, 76],
         [ 47.40809513255954, 8.552019177004695, 446.6000061035156, "2015-11-13T12:58:42.000Z", 27.0, 73],
         [ 47.408130671828985, 8.552070977166295, 446.20001220703125, "2015-11-13T12:58:46.000Z", 27.0, 70],
         [ 47.40816361270845, 8.552097212523222, 446.0, "2015-11-13T12:58:49.000Z", 27.0, 71],
         [ 47.40818230435252, 8.552138702943921, 445.6000061035156, "2015-11-13T12:58:51.000Z", 27.0, 71],
         [ 47.40825707092881, 8.552154125645757, 445.3999938964844, "2015-11-13T12:58:58.000Z", 27.0, 70],
         [ 47.40829017944634, 8.55216016061604, 445.3999938964844, "2015-11-13T12:59:01.000Z", 27.0, 71],
         [ 47.40830333903432, 8.552153706550598, 445.20001220703125, "2015-11-13T12:59:03.000Z", 27.0, 71],
         [ 47.408308032900095, 8.552153706550598, 445.20001220703125, "2015-11-13T12:59:04.000Z", 27.0, 70],
         [ 47.40834197960794, 8.552186479791999, 445.3999938964844, "2015-11-13T13:01:07.000Z", 27.0, 85],
         [ 47.40833795629442, 8.552188323810697, 445.3999938964844, "2015-11-13T13:01:08.000Z", 27.0, 84],
         [ 47.40833401679993, 8.552198549732566, 445.3999938964844, "2015-11-13T13:01:09.000Z", 27.0, 84],
         [ 47.40832010284066, 8.552253786474466, 445.3999938964844, "2015-11-13T13:01:12.000Z", 27.0, 84],
         [ 47.40828674286604, 8.552344981580973, 445.0, "2015-11-13T13:01:17.000Z", 27.0, 80],
         [ 47.40825455635786, 8.552448330447078, 444.79998779296875, "2015-11-13T13:01:23.000Z", 27.0, 79],
         [ 47.40825262852013, 8.552489904686809, 444.79998779296875, "2015-11-13T13:01:26.000Z", 27.0, 80],
         [ 47.40821675397456, 8.55262778699398, 444.79998779296875, "2015-11-13T13:01:35.000Z", 27.0, 79],
         [ 47.40820610895753, 8.552680592983961, 444.6000061035156, "2015-11-13T13:01:38.000Z", 27.0, 79],
         [ 47.40816168487072, 8.552955100312829, 444.6000061035156, "2015-11-13T13:01:52.000Z", 27.0, 76],
         [ 47.408138466998935, 8.553202534094453, 444.20001220703125, "2015-11-13T13:02:03.000Z", 27.0, 76],
         [ 47.408125810325146, 8.553239582106471, 444.0, "2015-11-13T13:02:05.000Z", 27.0, 75],
         [ 47.40811625495553, 8.553247042000294, 443.79998779296875, "2015-11-13T13:02:06.000Z", 27.0, 75],
         [ 47.40810167044401, 8.553252154961228, 443.6000061035156, "2015-11-13T13:02:07.000Z", 27.0, 76],
         [ 47.40808733738959, 8.553255340084434, 443.6000061035156, "2015-11-13T13:02:08.000Z", 27.0, 77],
         [ 47.408021707087755, 8.553288113325834, 444.20001220703125, "2015-11-13T13:02:13.000Z", 27.0, 80],
         [ 47.4079795461148, 8.55328886769712, 444.6000061035156, "2015-11-13T13:02:17.000Z", 27.0, 82],
         [ 47.40796043537557, 8.553284592926502, 444.79998779296875, "2015-11-13T13:02:19.000Z", 27.0, 83],
         [ 47.40791064687073, 8.553258189931512, 445.20001220703125, "2015-11-13T13:02:23.000Z", 26.0, 86],
         [ 47.40785448811948, 8.553215442225337, 445.6000061035156, "2015-11-13T13:02:27.000Z", 26.0, 89],
         [ 47.40780486725271, 8.553174622356892, 446.20001220703125, "2015-11-13T13:02:32.000Z", 26.0, 92],
         [ 47.40777527913451, 8.553162636235356, 446.79998779296875, "2015-11-13T13:02:35.000Z", 26.0, 94],
         [ 47.40773663856089, 8.553122906014323, 447.3999938964844, "2015-11-13T13:02:39.000Z", 26.0, 96],
         [ 47.40771467797458, 8.553109411150217, 447.6000061035156, "2015-11-13T13:02:41.000Z", 26.0, 98],
         [ 47.407703530043364, 8.55310220271349, 448.0, "2015-11-13T13:02:42.000Z", 26.0, 98],
         [ 47.407659692689776, 8.553062472492456, 448.79998779296875, "2015-11-13T13:02:46.000Z", 26.0, 101],
         [ 47.40764569491148, 8.553053420037031, 449.0, "2015-11-13T13:02:47.000Z", 26.0, 102],
         [ 47.40763236768544, 8.553045960143209, 449.20001220703125, "2015-11-13T13:02:48.000Z", 26.0, 103]
       ]
     ]
   } }
] }

http://solveig.oslo.no/anchoring-jtt.json ... is this valid JTT?

hard to tell @tobixen, since there is no format definition to validate against. looking at this 6 years later i definitely wouldn't create this "spreadsheet-like" structure with "heading row" with field names and then all "data rows" following that structure. that seems to invite a lot of brittleness.