search

bfast2 / bfast

Breaks For Additive Season and Trend
https://bfast2.github.io
GNU General Public License v2.0
41 stars 15 forks source link

Stable period in the BFAST Lite algorith: Benefits and drawbacks #112

Closed nikosGeography closed 2 weeks ago

nikosGeography commented 3 months ago

This is not a coding question but more of a theoretical one, so if it doesn't belong here feel free to close it.

I am about to use the BFAST Lite algorithm to perform a satellite time-series analysis (STSA). One of the parameters I have to specify is the lenght of the stable period (a time period within the time series data that is assumed to be free from significant structural changes).

The data I am about to use are monthly satellite nighttime lights (NTL) for the years 2013 to 2023. From a basic visual inspection, I saw that during 2014, there was a brightening in my study area and during the 2019 a dimming (due to COVID?). The issue is that I wanted to to use the years 2013 to 2018 as a stable period, but now I can't.

If I use them, the algorithm doesn't identify a break in 2019 (I haven't tested the behaviour of the method if I lessen the length of the stable period).

My question is: What are the benefits and drawbacks of a smaller stable period in the BFAST Lite method? Is there a recommended time length? I'm thinking to use data for the period 2018-2023.

Thank you.

GreatEmerald commented 2 weeks ago

I'm not sure if I understand what you mean exactly. The stable period definition is only relevant for BFAST Monitor, not BFAST Lite. In BFAST Lite, you specify the shortest segment size (h parameter), i.e. minimum time between breakpoints. The breakpoints themselves get detected automatically.

nikosGeography commented 2 weeks ago

I am sorry for not making the question more clear. First of all, the time I asked the question I thought that I had to specify a stable period in Lite just like in the "classical" BFAST. That's not the case, as you mentioned and as I learned by now.

GreatEmerald commented 2 weeks ago

To be exact, you only need to specify a stable period in BFAST Monitor. There is no difference between BFAST and BFAST Lite in that regard. It is also not needed for BFAST01.