# Thermal assessment of redevelopment in a school playground by UAV with a non-radiometric IR camera and on-site measurements

Rémy Claverie, Julien Bouyer, Nassima Hassani, Luc Manceau, Gilles Drogue

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Rémy Claverie, Julien Bouyer, Nassima Hassani, Luc Manceau, Gilles Drogue. Thermal assessment of redevelopment in a school playground by UAV with a non-radiometric IR camera and on-site measurements. 12th International Conference on Urban Climate (ICUC), Jul 2025, Rotterdam, Netherlands. hal-05162299

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Submitted on 15 Jul 2025

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# Thermal assessment of redevelopment in a school playground by UAV with a non-radiometric IR camera and on-site measurements

Rémy Claverie<fnref n="1" />, Julien Bouyer<fnref n="1" />, Nassima Hassani<fnref n="2" />, Luc Manceau<fnref n="2" />, and Gilles Drogue<fnref n="2" />

## Background

Children are considered more sensitive and vulnerable to hot ambient conditions compared to adults, and are affected strongly by their thermal environment (Vanos et al., 2015). One of the urban outdoor environments that children spend almost one third of their school time is the schoolyard. Schoolyards in urban areas are usually made of asphalt and other paved materials, and are characterized by the absence of trees. Based on a literature survey, Antoniadis et al. (2020) reported that "typical urban schoolyard design (i.e., dense surface materials, absence of trees) triggered high surface temperatures (that may exceed 58 °C) and increased absorption of radiative heat load (that may exceed 64 °C in terms of Mean Radiant Temperature) during a clear day with intense solar radiation."

In order to mitigate this "schoolyard heat island" which could represent a significant total area in a city complex, public authorities need fast and inexpensive expertise. For this, it is necessary to test simple but not "simplistic" measurement protocols for establishing micro-climatic diagnosis and comparison of the evidence before and after green infrastructure interventions (Koc et al., 2018).

In this study, we test the possible link between outdoor thermal comfort measures recorded in a school playground and the IR signal from a non-radiometric FLIR UAV thermal camera (Kelly et al). To fit the relation between digital numbers and temperature values for images captured during UAV flight, the contact temperature of three targets with different thermal properties are measured when the UAV thermal camera shots the scene. We additionally explore the link between UAV IR signal and the Mean Radiant Temperature, asking whether IR can be used as an explanatory variable for thermal comfort. We also investigate the possibility of making a before-and-after diagnosis using a single day's measurement, rather than using multi-date continuous monitoring. To this end, we test the hypothesis that measuring surface temperature would enable us to directly observe the effect of rearranging the ground surface.

## Surface temperature assessment

<figure><img src="image_2.png" /><figcaption>Calculation of radiometric parameter from IR temperatures of targets and Flir equation</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>White target: Surface temperature is centered on 27.81°C</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Gray target: Surface temperature is centered on 41.711°C</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Black target: Surface temperature is centered on 62.32°C</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>IR thermal targets</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>DN of the scene</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fit of T<sub>s</sub> with the three targets</figcaption></figure>

## Objectives

**General objective:** to propose a low-cost and easy-to-implement method for analyzing the microclimate and comfort in public spaces planned for renovation.

But also:

* Deploy a new methodology for surface temperature measurements from a UAV associated with a non-radiometric IR camera;

* Confront teledetected surface temperatures with micoclimatic measurements at the ground level;

* Exhibit thermal comfort zones in a schoolyard, and explain the impact of the vegetation.

## Experimentation site

Saint-Eucaire school is located in historic center (LCZ 2) of Metz city, north-east of France, 390,000 inhabitants in its urban unit, semi-continental (or degraded oceanic climate) of the central and northern plains.

<figure><img src="image_9.png" /><figcaption>Aerial view of Saint-Eucaire school yard</figcaption></figure>

Microclimatic and comfort measurements

Ground IR measurements (validation)

IR thermal targets

## Schoolyard ST map

<figure><img src="image_10.png" /><figcaption>Surface temperatures derived by algorithm</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Differences with a linear correction</figcaption></figure>

## Instruments

### UAV (drone)

* DJI Phantom 4 Pro

### Camera: FLIR Vue Pro

* Spectral band: 7.5 - 13.5 µm

* Array: 336x256

### Reference weather station

* Davis Vantage Pro 2

### Comfort station

* Temperature / hygrometry (HygroVUE 5)

* Black globe temperature (Campbell Scientific -L)

* Wind speed / direction (Campbell Scientific 03002)

* Net Radiometer (Kipp & Zonen CNR4)

* Datalogger: CR1000x

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

## Microclimatic measurments

### Measurement campaign

July 21, 2021 from 10h UTC to 15h30 UTC

Hourly measurement loops

Stop (5 min) an go from spot 1 to 9 (+)

<figure><img src="image_14.png" /><figcaption>Air temperature measurments for each locations</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>Wind speed measurments for each locations</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>Incident solar radiation measurments for each locations</figcaption></figure>

## Results

* We have optimized a low-cost method for using a non-radiometric drone to analyze the thermal environment of narrow urban spaces

* Although deeper analyses are currently being carried out, we have built up an interesting dataset (1 summer day in 2021 & in 2023) to confront microclimatic and comfort measurements in the urban canopy with radiative measurements from the contributing surface.

* Initial analyses have already identified thermal gradients in the courtyard:

* only 1°C variability in air temperature, but more than 5°C for UTCI;

* mainly due to the stocking coatings on the floor;

* reinforced by increased proximity to the facades of adjacent school buildings.

## Future work

* First, we will use the 1st « AFTER » dataset (2023) to evaluate the thermal comfort differences cause by the refurbishment.

* In 2026, we will carry out the same measurement campaigns with the addition of another drone equipped with an IR-radiometric camera. This will enable us to re-examine the methodology and establish a comparative analysis of digital images and thermal images.

* All of the measurements taken (2021, 2023, 2026) will enable us to establish the impact of the improvements over seven years, particularly the impact of trees on thermal comfort.

* We plan to model surface temperatures based on the radiation from the facades in order to better assess thermal comfort in the schoolyard.

## References

Antoniadis et al., (2020). *Thermal Environment of Urban Schoolyards: Current and Future Design with Respect to Children's Thermal Comfort*. Atmosphere, 11(11), 1144; https://doi.org/10.3390/atmos11111144

Kelly, J., N. Kljun, P.-O. Olsson, L. Mihai, B. Liljeblad, P. Weslien, L. Klemedtsson, and L. Eklundh, 2019: Challenges and Best Practices for Deriving Temperature Data from an Uncalibrated UAV Thermal Infrared Camera. Remote Sensing, 11, 567, https://doi.org/10.3390/rs11050567

Koc et al. (2018). Evaluating the cooling effects of green infrastructure: A systematic review of methods, indicators and data sources. https://doi.org/10.1016/j.solener.2018.03.008

Vanos J.K., 2015. *Children's health and vulnerability in outdoor microclimates: A comprehensive review*. Environment International, 76, 1-15. https://doi.org/10.1016/j.envint.2014.11.016

## Contacts

remy.claverie@cerema.fr

<footnote n="1">¹Cerema, TEAM Research unit, Nancy, France</footnote>

<footnote n="2">² Université de Lorraine, LOTERR, F-57000 Metz, France</footnote>