Author Contributions
Conceptualization, L.M.; Data curation, L.M.; Formal analysis, L.M.; Funding acquisition, L.M. and S.P.; Methodology, L.M.; Project administration, S.P.; Resources, S.P.; Software, L.M.; Validation, Lonesome Malambo; Writing—original draft, L.M.; Writing—review & editing, L.M. and S.P. All authors have read and agreed to the published version of the manuscript.
Figure 1.
Ground track (GT) naming convention: (
a) Advanced Topographic Laser Altimeter System (ATLAS) oriented in the forward (instrument coordinate +x) direction, (
b) ATLAS oriented in the backward (instrument coordinate +x) direction. Image credit: ICESat-2 Project Team [
9].
Figure 1.
Ground track (GT) naming convention: (
a) Advanced Topographic Laser Altimeter System (ATLAS) oriented in the forward (instrument coordinate +x) direction, (
b) ATLAS oriented in the backward (instrument coordinate +x) direction. Image credit: ICESat-2 Project Team [
9].
Figure 2.
Interpretation of ATL03 data. Observing point distribution differences is useful in interpreting noise (random) and signal (clustered) points: (a) Sample ATL03 data with lower noise levels over a site in north-western Zambia; (b) Sample ATL03 data with higher noise level over a site in eastern Texas, United States; (a,b) also show further classification of signal points into terrain (lower local elevation variation) and vegetation (above terrain and exhibit higher local elevation variation); (c) Sample ATL03 data corrupted by instrument errors, which could be interpreted as noise. (d) Samples data with cloud points, which are identifiable by their significant elevation values compared to terrain points; (e) Interpretation of ATL03 data over a built area in Houston, Texas. Shape attributes and regularity of surface is key to differentiating them from terrain; (f) Using ancillary image data to aid interpretation of data in (e). ATD is along-track distance.
Figure 2.
Interpretation of ATL03 data. Observing point distribution differences is useful in interpreting noise (random) and signal (clustered) points: (a) Sample ATL03 data with lower noise levels over a site in north-western Zambia; (b) Sample ATL03 data with higher noise level over a site in eastern Texas, United States; (a,b) also show further classification of signal points into terrain (lower local elevation variation) and vegetation (above terrain and exhibit higher local elevation variation); (c) Sample ATL03 data corrupted by instrument errors, which could be interpreted as noise. (d) Samples data with cloud points, which are identifiable by their significant elevation values compared to terrain points; (e) Interpretation of ATL03 data over a built area in Houston, Texas. Shape attributes and regularity of surface is key to differentiating them from terrain; (f) Using ancillary image data to aid interpretation of data in (e). ATD is along-track distance.
Figure 3.
Web map integration. The section of an ICESat-2 track under consideration is displayed on a web-based map to aid interpretation of the ATL03 data.
Figure 3.
Web map integration. The section of an ICESat-2 track under consideration is displayed on a web-based map to aid interpretation of the ATL03 data.
Figure 4.
PhotonLabeler graphical user interface showing track and label information and tools on the left panel, plot areas in middle and data display control and labeling and navigation tools on the right panel.
Figure 4.
PhotonLabeler graphical user interface showing track and label information and tools on the left panel, plot areas in middle and data display control and labeling and navigation tools on the right panel.
Figure 5.
Label definition tools in the PhotonLabeler. The red arrow illustrates the process of defining a new point label.
Figure 5.
Label definition tools in the PhotonLabeler. The red arrow illustrates the process of defining a new point label.
Figure 6.
Manual Photon Labeling using the Photon Labeler. (a,b) Label sections along ICESat-2 tracks for the Zambian and Texas sites; (c,d) Close-up plot and base map views of 100-m segments for the Zambia and Texas sites.
Figure 6.
Manual Photon Labeling using the Photon Labeler. (a,b) Label sections along ICESat-2 tracks for the Zambian and Texas sites; (c,d) Close-up plot and base map views of 100-m segments for the Zambia and Texas sites.
Figure 7.
Comparison of PhotonLabeler and ATL08 photon label data for the Zambia and Texas sites over 300 m distance: (a) ATL03 data for Zambia site with photon labels from PhotonLabeler (0 = Noise, 1 = Terrain, 2 = Off-terrain), (b) ATL03 data for Zambia site with photon labels from ATL08 product (0 = Noise, 1 = Terrain, 2 = Off-terrain), (c) Photon class disparity for the Zambia site showing regions of disagreement (0,red) and areas of agreement (1, black). (d) ATL03 data for Texas site with photon labels from PhotonLabeler (0 = Noise, 1 = Terrain, 2 = Off-terrain), (e) ATL03 data for Texas site with photon labels from ATL08 product (0 = Noise, 1 = Terrain, 2 = Off-terrain), (f) Photon class disparity for the Texas site showing regions of disagreement (0, red) and areas of agreement (1, black).
Figure 7.
Comparison of PhotonLabeler and ATL08 photon label data for the Zambia and Texas sites over 300 m distance: (a) ATL03 data for Zambia site with photon labels from PhotonLabeler (0 = Noise, 1 = Terrain, 2 = Off-terrain), (b) ATL03 data for Zambia site with photon labels from ATL08 product (0 = Noise, 1 = Terrain, 2 = Off-terrain), (c) Photon class disparity for the Zambia site showing regions of disagreement (0,red) and areas of agreement (1, black). (d) ATL03 data for Texas site with photon labels from PhotonLabeler (0 = Noise, 1 = Terrain, 2 = Off-terrain), (e) ATL03 data for Texas site with photon labels from ATL08 product (0 = Noise, 1 = Terrain, 2 = Off-terrain), (f) Photon class disparity for the Texas site showing regions of disagreement (0, red) and areas of agreement (1, black).
Figure 8.
Relationships between ATL08 minimum (Min), mean (Mean), and maximum (Max) height estimates and matching height estimates from PhotonLabeler (PL) labeled data for the Zambian (first column) and Texas (second column) sites: (a,b) Absolute canopy heights; (c,d) Relative canopy heights; (e,f) Terrain heights. The dashed line shows the expected 1:1 relationship between PL and ATL08 estimates.
Figure 8.
Relationships between ATL08 minimum (Min), mean (Mean), and maximum (Max) height estimates and matching height estimates from PhotonLabeler (PL) labeled data for the Zambian (first column) and Texas (second column) sites: (a,b) Absolute canopy heights; (c,d) Relative canopy heights; (e,f) Terrain heights. The dashed line shows the expected 1:1 relationship between PL and ATL08 estimates.
Table 1.
ATL08 height metrics evaluated for the study. Height metrics include the minimum (Min), mean (Mean), maximum (Max), and height percentiles (Pxx, e.g., P25 for 25th percentile height). Metrics evaluated for a particular height type (Absolute canopy, Relative canopy, and Terrain height) are marked by x.
Table 1.
ATL08 height metrics evaluated for the study. Height metrics include the minimum (Min), mean (Mean), maximum (Max), and height percentiles (Pxx, e.g., P25 for 25th percentile height). Metrics evaluated for a particular height type (Absolute canopy, Relative canopy, and Terrain height) are marked by x.
Metric | Max | Mean | Min | P25 | P50 | P60 | P70 | P75 | P80 | P85 | P90 | P95 |
---|
Absolute canopy height | x | x | x | x | x | x | x | x | x | x | x | x |
Relative canopy height | x | x | x | x | x | x | x | x | x | x | x | x |
Terrain height | x | x | x | | | | | | | | | |
Table 2.
Number of manually labeled points in northwestern Zambia and Texas.
Table 2.
Number of manually labeled points in northwestern Zambia and Texas.
| | Northwestern Zambia | Eastern Texas, USA |
---|
Label | Label Code | No. Points | Proportion (%) | No. Points | Proportion (%) |
---|
Noise | 0 | 695 | 3.2 | 7355 | 31.1 |
Terrain | 1 | 13,843 | 63.7 | 3306 | 14.0 |
Off-terrain | 2 | 7201 | 33.1 | 13,026 | 55.0 |
Total | | 21,739 | 100 | 23,687 | 100.0 |
Table 3.
Confusion matrix between PhotonLabeler and ATL08 photon labels for the Zambia site. Overall accuracy 95.4%.
Table 3.
Confusion matrix between PhotonLabeler and ATL08 photon labels for the Zambia site. Overall accuracy 95.4%.
| PhotonLabeler | Error (%) |
---|
| Labels | Noise | Terrain | Off-Terrain | Total | Commission | Omission |
---|
ATL08 | Noise | 380 | 22 | 9 | 411 | 7.5 | 45.3 |
Terrain | 215 | 13,693 | 531 | 14,439 | 5.2 | 1.1 |
Off-terrain | 100 | 128 | 6661 | 6889 | 3.3 | 7.5 |
Total | 695 | 13,843 | 7201 | 21,739 | | |
Table 4.
Confusion matrix between PhotonLabeler and ATL08 photon labels for the Texas site. Overall accuracy 95.4%.
Table 4.
Confusion matrix between PhotonLabeler and ATL08 photon labels for the Texas site. Overall accuracy 95.4%.
| PhotonLabeler | | Error (%) |
---|
| Labels | Noise | Terrain | Off-Terrain | Total | Commission | Omission |
---|
ATL08 | Noise | 6450 | 0 | 83 | 6533 | 1.3 | 12.3 |
Terrain | 644 | 3248 | 549 | 4441 | 26.9 | 1.8 |
Off-terrain | 261 | 58 | 12,394 | 12,713 | 2.5 | 4.9 |
Total | 7355 | 3306 | 13,026 | 23,687 | | |
Table 5.
Correlation (R2) between ATL08 absolute canopy, relative canopy, and terrain height metrics with metrics derived from manually labeled point data for the Zambia and Texas sites. Height metrics include the minimum (Min), mean (Mean), maximum (Max), and percentiles (Pxx, e.g., P25 for 25th percentile height).
Table 5.
Correlation (R2) between ATL08 absolute canopy, relative canopy, and terrain height metrics with metrics derived from manually labeled point data for the Zambia and Texas sites. Height metrics include the minimum (Min), mean (Mean), maximum (Max), and percentiles (Pxx, e.g., P25 for 25th percentile height).
| Absolute Canopy Height | Relative Canopy Height | Terrain Height |
---|
Metric | Texas | Zambia | Texas | Zambia | Texas | Zambia |
---|
Max | 0.94 | 1.00 | 0.82 | 0.60 | 0.99 | 1.00 |
Mean | 0.96 | 0.99 | 0.82 | 0.82 | 1.00 | 1.00 |
Min | 0.98 | 1.00 | 0.29 | 0.00 | 0.99 | 1.00 |
P25 | 0.87 | 0.98 | 0.57 | 0.67 | | |
P50 | 0.89 | 0.95 | 0.62 | 0.86 | | |
P60 | 0.90 | 0.94 | 0.71 | 0.79 | | |
P70 | 0.94 | 0.92 | 0.75 | 0.73 | | |
P75 | 0.96 | 0.93 | 0.79 | 0.78 | | |
P80 | 0.97 | 0.95 | 0.85 | 0.78 | | |
P85 | 0.98 | 0.97 | 0.84 | 0.79 | | |
P90 | 0.97 | 0.99 | 0.87 | 0.80 | | |
P95 | 0.97 | 0.99 | 0.89 | 0.76 | | |
Table 6.
Precision (mean biases) between ATL08 absolute canopy, relative canopy and terrain height metrics with metrics derived from manually labeled point data for the Zambia and Texas sites. Height metrics include the minimum (Min), mean (Mean), maximum (Max), and percentiles (Pxx, e.g., P25 for 25th percentile height).
Table 6.
Precision (mean biases) between ATL08 absolute canopy, relative canopy and terrain height metrics with metrics derived from manually labeled point data for the Zambia and Texas sites. Height metrics include the minimum (Min), mean (Mean), maximum (Max), and percentiles (Pxx, e.g., P25 for 25th percentile height).
| Absolute Canopy Height | Relative Canopy Height | Terrain Height |
---|
Metric | Texas | Zambia | Texas | Zambia | Texas | Zambia |
---|
Max | 4.29 | 0.35 | 4.63 | −0.17 | 0.45 | 0.19 |
Mean | −1.51 | −5.89 | 2.91 | 0.26 | −0.08 | 0.02 |
Min | −2.39 | −1.31 | 0.48 | 0.18 | −0.61 | −0.21 |
P25 | −5.13 | −6.36 | 3.14 | 0.70 | | |
P50 | −1.76 | −9.63 | 3.27 | 0.14 | | |
P60 | −0.61 | −9.76 | 3.16 | 0.03 | | |
P70 | 0.39 | −7.89 | 3.00 | −0.02 | | |
P75 | 0.90 | −6.44 | 3.00 | −0.10 | | |
P80 | 1.47 | −4.93 | 3.02 | −0.25 | | |
P85 | 1.83 | −3.31 | 3.12 | −0.33 | | |
P90 | 2.33 | −2.14 | 3.19 | −0.36 | | |
P95 | 2.69 | −1.33 | 3.36 | −0.34 | | |