Preface: the 2021 edition of the XXIVth ISPRS congress

Abstract. We report key elements and figures related to the proceedings of the 2021 edition of the XXIVth ISPRS Congress. Similarly to 2020, the COVID-19 pandemic caused global travel challenges and restrictions for the first half of 2021. Consequently, the physical Congress re-scheduled from June 2020 to July 2021 was again postponed to June 2022, still in Nice (France). Papers were already submitted and the ISPRS Council decided to carry out the review process and the publication of the proceedings of the papers submitted under the label “2021 Edition”. The authors of published papers had the opportunity to present their work during a Digital Event, this year scheduled the same week as the planned Congress (5–9 July 2021).



INTRODUCTION
We report key elements and figures related to the proceedings of the 2021 edition of the XXIV th ISPRS Congress.Similarly to 2020, the COVID-19 pandemic caused global travel challenges and restrictions for the first half of 2021.Consequently, the physical Congress re-scheduled from June 2020 to July 2021 was again postponed to June 2022, still in Nice (France).Papers were already submitted and the ISPRS Council decided to carry out the review process and the publication of the proceedings of the papers submitted under the label "2021 Edition".The authors of published papers had the opportunity to present their work during a Digital Event, this year scheduled the same week as the planned Congress (5-9 July 2021).

KEY ELEMENTS
The International Program Committee (IPC) established in 2020 was kept in order to prepare and implement this 2021 edition.The IPC includes the Congress Director, the ISAC Chair, the Program Chairs, the Chair of the ISPRS Student Consortium, Technical Commission Presidents and Vice Presidents (TCP).In order to efficiently handle the large expected amount of papers related to the Thematic sessions (see below), Michael Ying Yang was nominated as Thematic Session Chair and integrated the IPC.The templates for paper submission (both abstracts and full papers) were refined in order to clarify the review process of abstracts and ease the publication of the accepted papers.They are available on the ISPRS website.

Tracks & submission process
Authors had the possibility to submit their work through different tracks: • Technical Commission tracks (5): one track for each Technical Commission, managed by the TCP and with topics corresponding to the TC Working Groups (WG); • Youth Forum: managed by the ISPRS Student Consortium; The submission and the review processes of each TC were monitored by seven TC Correspondents (1 for each TC, except for TCIII with 2), that were also dedicated to help TCP and WG officers with the Conference Managing System (Conftool).

Important dates
The main drawback of large events is the significant time lapse between paper submission and the event.We reduced the global time frame mainly by minimising the reviewing period.This allowed to shift the submission deadline by more than one month without steadily modifying the notification date.As a consequence, authors of accepted abstracts have sufficient time to extend their paper.
• 4 February: Deadline for abstracts & full papers; • 7 March: Notification of authors for abstracts; • 5 March: Notification of authors for full papers; • 22 April: Deadline for camera ready papers.

Thematic sessions
Thematic Sessions (TS) were created for the 2020 edition in order to promoting emergent and cross-discipline topics not covered by the ISPRS Working Groups (Mallet et al., 2020).19 topics were selected among the 2020 TS and new proposals.They are listed in Table 1.Same deadlines and formats applied as for the TC tracks.Several of them only welcomed invited papers.
Each TS was linked to a specific TC.Final papers are published on the Volumes corresponding to this TC.

Organisation
The overall workflow is described in (Mallet et al., 2018).Depending on the number of papers, TCPs either directly handled themselves the papers of their commission (TC I, II, V and Youth Forum), or they decided to involve Area Chairs for reviewer assignment and decision taking (TC III and IV).Area Chairs were selected among Working Group officers.Thematic Session organisers directly acted as Area Chairs under the supervision of the Thematic Session chair.In order to preserve the double-blind pee-review process for full papers and to guarantee objectivity in decision taking, we adopted the following strategy: • Papers co-authored by TCPs and Area Chairs were directly handled by the Program Chairs; • Papers co-authored by TS organisers were handled by another organiser or by the Thematic Session Chair.
Again, the IPC decided to use the "Conditionally Accepted" status for abstracts without sufficient evidence of scientific quality.This let the possibility to reject them if the cameraready paper did not sufficiently take reviewers' remarks into account (see below for the detailed statistics).Many authors with this status provided a rebuttal letter with their camera-ready paper to explain how they extended and improved their contribution.87 papers were under this status.After the withdrawal of 11 papers, 13 were eventually rejected (17%) and the other ones published.

Plagiarism detection
All accepted papers were scrutinised by the iThenticate software in order to detect cases of plagiarism.The software provides a full report for each paper.In particular, it computes a similarity score by comparing the contribution with iThenticate proprietary database, databases of other content providers, and documents retrieved through standard Internet search.A global similarity score is retrieved by agglomerating individual matching scores.High scores corresponded to either a strong overlap with preprints (which does not violate the ISPRS policy on preprints) or with journal papers.In the latter case, authors withdrew their contribution.

Statistics
667 papers were submitted (431 abstracts and 236 full papers).620 were conditionally accepted (92.9%) and 466 were eventually published (69.8%).This initially corresponds to 268 abstracts (62.1%) and 198 full papers (84%).353 papers are published in 5 volumes of the ISPRS Archives while 113 are published in the ISPRS Annals (57.1% of the published full papers, 24.4% of the published papers and 17% of the submitted papers).We notice a slight decrease with respect to 2020.Due to the announcement of a 2021 edition, we registered multiple withdrawals, either officially or unofficially (no upload of camera-ready papers).The trend remains less impactful than in 2020 (18.5% of the submitted papers instead of 30.5).This corresponded to 122 papers (106 abstracts and 16 full papers).
We collected 2.1 reviews per paper in average (2.0 reviews for abstracts and 2.4 reviews for full papers).Again, the evaluation criteria, leading to a score between 0 and 100, allowed to capture the main strengths and weaknesses of the submitted contribution and contributed to smoothly discriminate papers that should be rejected, accepted to the Archives or the Annals (Figure 3).

Young Author's Award
Based on the review process, each Technical Commission selected one paper for this award.The awardees are: TC I: Sensor systems • Jiayuan Li, Yongjun Zhang, Qingwu Hu (CN) for "Robust estimation in robot vision and photogrammetry: A general model and its applications".
TC IV: Spatial Information Science

Outstanding Reviewers
The Technical Commissions recognized the following reviewers as "Outstanding Reviewers" for their thorough reviews and deep involvement in the process: • TC I: Jan Skaloud (EPFL, CH).

Figure 3 .
Figure 3. Average review scores for the submitted papers.

Table 1 .
Thematic Sessions of the 2021 edition of the ISPRS Congress."-" means no papers were published in the proceedings.

Table 2 .
Detailed statistics for each track.