The meaning of "sparser" is that the observations were taken at greater intervals of time and/or space, i.e. further apart.

https://bit.ly/2IwZKFT
Philippe Combessie - 1996 - ‎Prisons
... d'autres fois, notamment lorsqu'il s'agit des réactions de la population, j'ai construit mes démonstrations à partir d'informations et d'observations plus éparses.

https://www.techno-science.net/actualite/el-ni-o-cours-holoc...
Dec 27, 2015 - El-Niño au cours de l'Holocène: observations et simulations ... de même type mais plus éparses dans le temps ou dans l'espace, à savoir que:


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Note added at 14 hrs (2019-10-08 10:13:31 GMT)
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When there is a cognate available, why not check to see whether it fits, rather than struggling to use something else?

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Note added at 14 hrs (2019-10-08 10:17:12 GMT)
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If you are wondering why I changed this from singular to plural, it is because I don't like the phrase "sparser observing system" and think it might be even less likeable in French. What was sparse was not so much the observing system, but the distribution of observation points within it and the observations that could be made.