Planning tips for in-field spectroradiometer measurements campaign

Collecting field data for a project is a tedious task. It is usualy a continuous process, which takes considerable amaunt of time and if something goes wrong we have to repeat the whole process again. Field data is also often the stepping stone for any following project activities, thus it should be provided on time and with good quallity.This is why planing in advance is crucial for every field campaing.

Quick intro

There are many common-sense rules for planning field work, however, this short article will focus on the specifications that apply to spectroradiometer measurements. Spectral measurements are used to cross-reference different satellite image products and to identify systematic errors in multitemporal data. We will  take as an example continuous spectroradiometer campaign done at the same time and date as Sentinel 2 image acquisitions.

Complete Workflow for a Spectroradiometer Field Campaign

A.      Prerequisites:

·       Install the software that comes up with your spectroradiometer ( in our case that is JETI Lival included in the USB of the spectrometer package)

·       Install SW Maps app on mobile phone

·       Choose AOI using open VHR images (e.g. ArcGIS basemap)

·       Overlap it with Sentinel 2 GRID 10m and 20m

·      Make sure

·       Import Sentinel 2 grid of AOI in SW Maps app

·       Make small field trips to explore the area, check if it is open to the public or a private land of farmers)

·       Get a calendar of dates when the satellite will pass

📖

We are using the SW map app in order to see our position in the field compared to the S2 grid. This is important because we want to make measurements only on homogeneous pixels, and not on mixed pixels. This should be considered also when choosing your AOI. It is very important to check if the AOI is publically avilable, or if it is a privite property for which we need to ask for permission!

P.S. You can also make photos with the SW app that are save with their corresponding location which comes in handy.

B.      Preparation in the field (35-40min)

1.      Open SW Maps app (with loaded Sentinel Grid files at your AOI)

2. Fill in protocol data about the environmental conditions

•        Date

•        Time start:

•        Time finish:

•        Clouds? 1-10 (photo SW Maps app) Check how to export photos

•        Heize? Yes/ No

•        Height of vegetation (photo SW Maps app)

•        Status of vegetation

•        Height of spectraval measurements

3.       Set the white panel to be ready for measurements (outside of the measurements zone)

4.       Turn on the spectroradiometer (and connect it to laptop if using one)

📖

I awlays take a lot of photos while doing my measurements. This helps, if you need to clarify anything to your team leader. For example how did you define the haze condition? You can show photos of haze and no haze images. Bellow you can see an image with small amounts of haze preasent. It is also very usefull to have some photos to include for a final report :)

C.       Take Measurements

1.       Take reference measurement of the white plate (rename it if working with laptop “white_plate_start”)

2.       Go to one of the corners of the selected pixel (check through SW Maps app)

3.       Start tracking on SW Maps

4.       Start continuous measurements and cover the pixel through walking

5.       Stop continouos measuremts and save them (as reference to be loaded in JETI software again)

6.       Stop SW tracking and save it with the current time and date

7.       Take a finish reference measurement of the white plate “white_plate_finish”

📖

You will take a big amount consequative measurements possily at different sites and in different dates. If you are working with a laptop at the time of measuring you can nicely organize everything in folders and use adequate naming conventions. This is not the case if you don't have a laptop with you (anyway using a laptop in the field is not recomended if you are working alone and there is no one else to help you handle all the equipment. So, names will be automatically assigned to your data files. How to destinguish between different AOI sites then? Simple note down the start and the end of each continuous or white plate measurement and where it was taken. You can then look at your notes at home and organize all your files.

D.      After the campaign

1.       Load results in JETI

2.       Create spectral library

3.       Export tracking data from SW Maps app to GeoJSON

4.       Export Sentinel 2 image for this day for AOI area

5.       Get spectral signature for TOA (in GEE)

Some Useful Tips

Arrive earlier

Make sure to be 30 to 40 minutes earlier on site. You will use this time to set your equipment ready, take notes on the weather condionts and make your preliminary measurements on the vegetation.

Wear black clothing

Spectroradiometers have very high spectral resolution and are thus very sensitive to changes in reflected light from the surroundings. For this reason, it is very important to conduct the measurements with black clothing and footwear. Black color has the property to absorb light rather than reflecting it and would not interfere with the measurements. The contrary would happen if you wear white t-shirt, blue pants and yellow shoes. There will be a mix of reflected light from yourself to the ground on which you do the measurements.

Long trousers and good shoes

Try to have your feet all cover, especially if you will walk in long grass. This will help against insects and potentially snakes.

Don’t forget a hat and water

When you are all covered up in black especially during the summer, you will definitely feel the heat and you might spend hours below the sun taking measurements. So it is very important to protect yourself with a hat and to stay hidrated.

Be quick!

The light can change very fast as the sun is moving, and in general it is good to spend not more than an hour on your measuremnets. It might happen that people passing by are interested in your work and it is great to explain what you are doin but awlays keep an eye on the time.