PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING

459

The difference between the two 1 m DEMs as well as its

histogram are shown in Figure 5. The majority of the area in

Figure 5a appears yellow, suggesting the elevation difference

ranges from -0.5 to 0.5 m. The histogram in Figure 5b further

reveals the peak difference occurs at a mean of 0.21 m, i.e.,

the SPL DEM is in general higher by this amount than the

reference DEM. Comparing the two shaded reliefs in Figure 4

and Figure 3 for the forest area, we notice that many small

fractal, rugged terrain features distributed rather evenly over

the study area. With reference to the image shown in Figure 2,

it is believed they are small bushes or trees remained in the

filtering results. The difference is likely caused by imperfect

filtering in forested areas where highly dense multi-stop data

is processed. We therefore see a challenging data processing

task for new, advanced lidar developments, eg., SPL.

We note that the ground point density of the SPL data after

filtering is as high as 3.6 points per sq. meter and the average

point spacing reaches 0.52 meters. Such dense points make

it theoretically possible to generate DEMs with a resolution

higher than 1 meter, except areas under dense trees. To

Figure 4. Point cloud (left), filtered ground points (middle) and the 1 m shaded DEM (right) from the linear lidar over the Forest area.

explore the potential of the SPL data, Figure 6 provides the

DEMs with 0.5 m, 0.25 m and 0.1 m resolutions. We can see

that the DEM with higher resolution presents less noise than

the lower ones. For example, the height difference along the

forest roads becomes less obvious as the resolution increases.

This can also be observed if we compare the SPL DEMs in

Figure 6 with the 1 m linear lidar DEM in Figure 3, where the

most significant differences can be found along the roads or

trails under canopy.

Finally, we would like to look into the distribution of the point

density. After filtering, the linear lidar dataset has 37,512

(31.51%) ground points left, corresponding to a point density

0.26 points per sq. meter at a ground spacing 1.96 meters.

When compared to the SPL Forest dataset in Figure 3 and

Table 3, the raw SPL data is 12.2 times (10/0.82) denser

as the linear lidar data, but its ground point density after

filtering is as 13.8 times (3.6/0.26) higher. However, it should

be noted that the distribution of remaining ground points is

not even, as depicted by the point density in Figure 7. For

comparative purpose, the figure also depicts the distribution

Figure 5. Difference of the SPL and linear lidar DEM (in meters) and its histogram of the Forest dataset.

(a)

(b)

SEO Version