topic Re: MODIS NDVI data missing attribute table in ArcGIS Pro Questions

MODIS NDVI data missing attribute table

KatiBurton — Tue, 29 Jun 2021 23:21:13 GMT

I am working on using MODIS images to compare NDVI values from 2000-2020 in the Inner Mongolian region of China. I am using 500m resolution 16 day images from the terra MODIS downloaded from AppEEARS. I downloaded the images and used the "extract by mask" tool and batch option to extract an NDVI raster for each year for a particular watershed boundary. From there I used the raster calculator to multiply each raster by 0.0001 (the scale factor of NDVI). Next I used raster calculator again to subtract each year from the previous year and generate a number of rasters showing the change in NDVI between each year from 2000-2020 (for example there is a raster showing changes in NDVI between 2000 and 2001, and 2001 and 2002, etc.). The problem I am having is that there are no attribute tables associated with these rasters. I have tried using "copy raster" to change the pixel type from floating 32 bit to 32 bit signed, but it changes all NDVI values to 0 for the copied raster. Even if I use "build raster attribute table" on the copied raster with a 32 bit signed pixel type, an attribute table does not appear after the tool runs. I need to know the areas associated with each increase or decrease in order to quantify the area of the watershed that increased or decreased in NDVI each year.

Re: MODIS NDVI data missing attribute table

DanPatterson — Tue, 29 Jun 2021 23:53:41 GMT

floating point rasters don't have tables

If you need a table you may need to scale up your values by some factor, then take the integer of that

Int(raster * 100 )

but you will have to remember that the values are scaled up

Re: MODIS NDVI data missing attribute table

KatiBurton — Wed, 30 Jun 2021 00:25:40 GMT

Thank you so much for your response!

Is this operation "Int(raster * 100)" something I can do using the raster calculator tool?

Re: MODIS NDVI data missing attribute table

DanPatterson — Wed, 30 Jun 2021 00:29:33 GMT

Yes it is, Int is a spatial analyst raster function and you would just select your raster to replace raster in the equation... give it a try

Re: MODIS NDVI data missing attribute table

KatiBurton — Wed, 30 Jun 2021 00:31:44 GMT

Fantastic! I just tried this and it worked and produced a table. The table gives me the NDVI values and the count, which I am assuming is how many times they appear in the raster. Do you know if there is a way to also calculate how much area each NDVI value occupies?

Re: MODIS NDVI data missing attribute table

DanPatterson — Wed, 30 Jun 2021 02:13:54 GMT

Your raster cell size times the number of cells is the area of each class/value

Re: MODIS NDVI data missing attribute table

KatiBurton — Wed, 30 Jun 2021 02:41:38 GMT

One last question to make sure I am understanding.

If my raster cell size is 463.312716527777 (shown in attached image) and the count for a value of 1 is 606 (shown in attached image), I need to multiply the two numbers together and the area would be the product with units of meters squared?

Re: MODIS NDVI data missing attribute table

DanPatterson — Wed, 30 Jun 2021 09:03:52 GMT

yes 463*463 * 606 and assuming units are meters

Re: MODIS NDVI data missing attribute table

KatiBurton — Wed, 30 Jun 2021 13:04:24 GMT

Thanks so much for all your help!