IEEE Geoscience and Remote Sensing Magazine - December 2023 - 128

specific research questions include the following three
questions: 1) exploring the temporal and geographic patterns
of TC research; 2) exploring the major research topics
and themes and their evolution over time; and 3) summarizing
the characteristics of TC method research in terms
of satellite sensors, DEMs, TC methods, evaluation criteria,
and applications.
METHODS
As research output continues to expand in terms of volume
and diversity and the availability of metadata about
research from literature databases such as Scopus and WoS,
academics have started examining trends in academic publications
[53], [54]. The systematic literature review of TC
has been structured as two distinct steps: 1) the literature
search and selection process and 2) the evaluation and interpretation
process.
LITERATURE SEARCH AND SELECTION
The literature search was conducted using two renowned
international academic literature databases [55], i.e., WoS
and Scopus, to assess the literature on TC. Research on specific
topics can be identified in these databases by conducting
systematic searches using a standard keyword and Boolean
operator, and the metadata linked to the publications were
downloaded for further analysis [54]. The topic and titleabstract-keywords
searches were respectively performed in
the WoS and Scopus databases using the following search
queries: " topographic correction " or " topographic normalization "
or " illumination correction " or " topographic effect
(s) " or " terrain correction " or " terrain normalization " (search
date: 1 January 1980 to 31 August 2022) (see Figure 2). The
query was limited to articles, reviews, conference papers, and
books. Meanwhile, the research direction for WoS and the
subject category for Scopus were limited to " Remote Sensing "
and " Earth and Planetary Sciences, " respectively. After the
query limits were applied, WoS and Scopus returned 755 and
5,164 publications respectively, resulting in a total of 5,919
publications. Metadata from these 5,919 publications were
downloaded from Scopus and WoS and then combined into
a bibliographic citation management tool, i.e., EndNote X9
[56]. After removing duplicates and manual screening, 426
publications highly related to the research topic were finally
selected for further analysis.
EVALUATION AND INTERPRETATION
A bibliometric analysis was first used to decipher and map
the cumulative scientific knowledge and evolutionary nuances
of well-established fields by making sense of large
volumes of unstructured data in rigorous ways [53]. The
temporal patterns of TC studies in the literature were first
identified. Then, to explore the geographic trends in the
research, the number of publications by country of author
affiliation and the locations of the studies were mapped
and compared to the global mountainous area delineated
by the Global Mountain Biodiversity Assessment (GMBA)
128
Mountain Inventory from [57]. The number of publications
by country of author affiliation and the location of
the studies were determined by text mining all contributing
authors and the study area sections of each publication,
respectively. The exact location, i.e., the longitude and latitude
of the research sites, is also recorded.
Then, the frequency distribution of elevations for all of
the study sites was calculated in Google Earth Engine (GEE)
[58], comparing it with those of mountain areas around the
world. To identify dominant and emerging research themes
as well as their changes, using VOSviewer (version 1.6.18),
a keyword co-occurrence analysis of the selected articles
was conducted. This analysis presents a visual qualitative
understanding of the structure and important topics
for TC studies. The VOSviewer is a bibliometric mapping
software tool developed by Van Eck et al. [59] based on the
smart local moving algorithm [60] and has been successfully
applied in bibliometric analysis [51], [61], [62]. In the
analysis, the counting method used was " full counting, "
and the subsequent steps involved the selection of the
" all keywords " option and setting the minimum number
of keywords occurrence to three, where 66 out of 1,026 total
keywords identified were included. Further modifications
were then done manually for the final keyword selection
by removing repeating identical terms like " topographic effects "
and " topographic effect. " Finally, the characteristics
of satellite sensors, DEMs, TC methods, evaluation criteria,
and applications were investigated based on the text mining
of each publication.
RESULTS
NUMBER AND SPATIAL-TEMPORAL DISTRIBUTION
OF TOPOGRAPHIC CORRECTION STUDIES
The number of TC studies has consistently increased between
1980 and 2022 (see Figure 3). This could be explained,
first, by the growing interest in studies regarding
mountainous ecosystems, and second, by the increased
availability of satellite data. The first data records were derived
from Landsat and SPOT, which were the most used
satellite data sources in TC studies (see the " Characteristics
of Satellite Sensors, Digital Elevation Model, Topographic
Correction Methods, Evaluation Criteria, and Applications "
section). The free and open data policies boosted TC studies.
For example, Landsat and ASTER GDEM became freely
available at the start of 2008 [63] and 2009, respectively,
resulting in a substantial increase in TC studies.
The spatial distribution of the number of studies showed
a large imbalance at a global scale (see Figure 4). More than
88% of TC studies focused on the Northern Hemisphere. Research
institutions corresponding to the selected articles are
distributed in 53 countries, mainly in Asia (accounting for
43.7%, especially in China), followed by Europe (23.7%,
especially in Spain and Italy), and North America (17.1%,
especially in the United States). Surprisingly, the number of
studies for North America and Europe are not comparatively
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