Hello everyone,
I have a question about calculating the NDVI for a forested area over a period of ten years. I would like to know if I should calculate the average NDVI for each year, or if I should calculate the NDVI for the same specific date each year, which should be more or less the same date.
Thank you.
Hi,
A great many depends on what type of forest you are dealing with, and how long the growing season is. I also don't know what kind of research you are doing and what the aim is, whereas I assume you want to study the conidences of a mixed stand in a temperate climate.
In that case, it is a good idea to limit yourself to the growing season only. Beyond the growing season, the trees have no leaves and you will not get any information about the condition of the tree, only about the spectral reflectance of the undergrowth.
I also do not know what data you have at your disposition, are these satellite images? If so, what resolution? Maybe you have UAV data. If so, I would also suggest that you consider masking the undergrowth to study the spectral reflectance of the tree crowns, e.g. with a mask based on LiDAR data or photogrammetric point clouds or DSM.
As for calculating the average for a year and the average for months, it all depends on the specifics of your research, if you want to monitor a large area and have limited computing power, the average for a year seems to be a good idea. Because you can find a year with a lower NDVI and only then analyse the data from that year.
If I were to do a stand condition analysis over 10 years using 10m resolution satellite data, I would do it like this: collect all the images and select those from the growing season period, then calculate the NDVI and calculate statistics for the relevant polygons e.g. forest clearcuts or hexagons. I would then create an NDVI graph to analyse the variability of this indicator. I would not calculate an average over the year or months, I would work on the values obtained and possibly only apply some smoothing to the graph. Climatic data can also be added to the graph to find periods of poorer stand condition and to locate the cause of such a condition, as well as to interpret how the stand regenerates or not in the period after the anomaly.
If you encounter problems with processing, I can help. I use and recommend R programming.
Best regards,
Jakub Miszczyszyn
Jakub Miszczyszyn
Hello Sir,
Thank you for your response and your clarification on the subject. To put you in context, I am working on a forest area covering an area of 100,000 hectares. The objective of this study is to monitor vegetation dynamics in this forest area using satellite imagery from 2000 to 2020. As for the satellite images, I have not yet determined which type I will be working with. Regarding NDVI calculations, I found that the majority of studies use the annual average NDVI value, while others choose a specific date. That's why I asked the question. I hope the idea is clear.
Sincerely,
Dear Salwa Mokrane,
The choice between these two approaches depends on your research objectives and the availability of suitable imagery. If you have access to multiple cloud-free images throughout each year, calculating the annual average NDVI may provide a more representative measure of the overall vegetation condition. However, if you have limited imagery available or if you specifically want to track changes at a consistent point in the growing season, using the same date each year would be more appropriate.
It's important to note that when using the same date approach, you should aim to select a date that falls within the peak growing season for your forested area, as this will typically provide the most accurate representation of the vegetation vigor and health.
Ultimately, the decision should be based on your research objectives, the availability and quality of imagery, and the specific characteristics of your study area.
Brook Legese Dadhe Hello Sir,
I understood your answer well. Thank you very much for your reaction.
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