Analysis of the water volume, length, total area and inundated area ofthe Three Gorges Reservoir, China using the SRTM DEM data

Y. WANG*{, M. LIAO{, G. SUN§ and J. GONG{{Center for Geographic Information Science and Department of Geography,

East Carolina University, Greenville, NC 27858, USA

{LIESMARS, Wuhan University, Wuhan, Hubei 430070, China

§Department of Geography, University of Maryland, College Park, MD 20742, USA

(Received 16 March 2005; in final form 18 March 2005 )

Using the Shuttle Radar Topography Mission (SRTM) digital elevation model

(DEM) data covering the region of the Three Gorges Reservoir, Changjiang,

China, we have computed the water volume, length, and total and inundated

areas of the reservoir, with the assumption that the water surface within the

reservoir is flat. When the reservoir’s surface water level is 175 m above the mean

sea level, the computed values may be comparable to the official data published

by the Chinese government.

1. Introduction

Approved by the National People’s Congress of China in 1992, the construction of

the Three Gorges Dam on the Changjiang started in 1993a (Changjiang, in Chinese,

means long river.) When completed in 2009, the reservoir will control floods and

assist river navigation, and generate near 20 000 MW of hydroelectricity. The dam

will be the largest hydroelectric dam in the world, and will definitely have great

social and environmental impacts in the dam region and surrounding areas becauseof the large scope of inundation and resettlement (e.g. Tian and Lin 1989, Dai 1994,

Sullivan 1995, Adams and Ryder 1998, Becker 1999, Murphy 2002, Bergman and

Renwick 2004, http://www.china-embassy.org/eng/zt/sxgc/default.htm, last accessed

in February 2005). In June 2003, the reservoir started to store water. Currently, the

water level is about 140 m above the mean sea level within the reservoir. Even

though there are many reports, books, scientific papers, governmental documents,

and news (newspapers and TV broadcasts) about the giant reservoir, most of the

reports are in Chinese. Also, detailed procedures and datasets used to create thereports may not be available to the general public. Thus, it will be of interest not

only to compute, in particular, the water volume, length, total area, and inundated

area of the reservoir, but also to provide the detailed information of the procedures

and datasets used in the computation.

To carry out the calculation, digital elevation model (DEM) data are needed.

Although the US Geographical Survey (USGS) global DEM, GTOPO30 (http://

edcdaac.usgs.gov/gtopo30/gtopo30.asp, last accessed in February 2005) has been

available since 1996, its coarse spatial resolution of 30 arcsecond or about

900 m6900 m makes the data not applicable because the width of most of the

*Corresponding author. Email: wangy@mail.ecu.edu

International Journal of Remote Sensing

Vol. 26, No. 18, 20 September 2005, 4001–4012

International Journal of Remote SensingISSN 0143-1161 print/ISSN 1366-5901 online # 2005 Taylor & Francis

http://www.tandf.co.uk/journalsDOI: 10.1080/01431160500176788

(original) river channel in the reservoir is much less than 900 m. The release of

DEMs from the Shuttle Radar Topography Mission (SRTM) at the end of 2003

provides better global elevation data (between 60u S and 60uN). The SRTM

consisted of a specially modified radar system that flew onboard the Space Shuttle

Endeavour during an 11-day mission in February 2000 (http://www2.jpl.nasa.gov/

srtm/index.html, last accessed in January 2005). The spatial resolution is 3 arcsecond

or about 90 m690 m for non-US territory. The elevation value is in the format of

the signed 16-bit integer. The data are in raster format and are organized in

individual tiles that cover 1u61u in latitude and longitude (ftp://edcftp.cr.usgs.gov/

pub/data/srtm/Documentation/, last accessed in December 2004). Thus, in this

study, the SRTM DEMs, coupled with ancillary data are used to delineate the

reservoir and surrounding regions, and to derive the volume, length, total area, and

inundated area at different surface water levels within the reservoir. Also,

descriptions of the study area, SRTM DEM data, voids of the DEMs, void-

removal methods, and assumptions made and procedures used in the calculation are

given.

2. Analytical approach

2.1 Study area and datasets

The Three Gorges is between the eastern edge of the Sichuan basin, surrounded by

high mountains and plateaus, and flatter riverine plain of the Middle and Lower

Changjiang Plain. In the Three Gorges region, the Changjiang is forced to flow

through a narrow, 150 km long, steep-walled valley no greater than 250 m in width,

much like sand in an hourglass. Water from spring snowmelt upstream and wet

summer monsoon rains can fill this narrow passageway with such great fluid flux

that river levels may easily rise 6 m over a 24-h period. After leaving the constricted

confines of the Gorges region, the Changjiang meanders sluggishly across the flat

surface of the Middle and Lower Changjiang Plain (Dai 1994, Bergman and

Renwick 2004). The Three Gorges Dam is located at Sandouping, Hubei Province,

and it is about 38 km upstream from Yichang City, Hubei Province. The reservoir

will span over 600 km upstream from the dam site, mostly within Chongqing

Municipality. The reservoir could reach and pass Chongqing City when it will be

filled at a surface water height of 175 m (above the mean sea level).

Tiles of the SRTM DEM data covering the reservoir and its surrounding areas are

downloaded (ftp://edcftp.cr.usgs.gov/pub/data/srtm/) and then mosaicked. Mosaic

of Landsat 7 data of paths/rows 125/039, 126/039, and 127/039 are used to verify the

location of the dam site (e.g. figure 1) on the DEMs and to assist the delineation of

the Changjiang and its tributaries and of areas on both banks impacted by the

reservoir. Figure 2 shows the delineated study area highlighted by the dotted lines.

Chongqing City and the dam are indicated. Yichang City (not shown in figure 2) is

downstream and further south-east from the dam site.

2.2 Voids of the DEM data

Since the SRTM DEMs are generated based on the radar interferometric technique,

there are missing elevation values or voids in the data. A void is set to have an

elevation value of 232 768 m. Figure 3, as an example, shows the DEM near the dam

site and the voids in black. The voids occur in shadow areas where there are no

backscatters to the radar, phase unwrapping anomalies, or other radar-specific

4002 Y. Wang et al.

causes (e.g. low coherence). For instance, there are some voids on the surface of

Changjiang due to low coherence from the moving water and low (to no)

backscattering from the smooth water surface. The space shuttle flew in a general

north–south direction, and the onboard radar looked perpendicularly to the flight

direction. If mountains are oriented roughly in north–south directions, and the

mountains have steep slopes (especially on the side facing away from the radar),

the gorges behind the mountains will be the shadow areas of the radar signals. The

higher the mountain and/or the steeper the slope, the larger the shadow area; two or

more voids are noticed (e.g. figure 3). Additionally, layover can produce voids. That

is a peculiarity of a side-looking radar and is like the opposite of shadowing. If a

mountain facing the radar illumination has a steeper angle than the radar incidence

angle, then the top of the mountain is imaged before the bottom of the mountain,

Figure 1. Landsat 7 band 8 image of the dam site acquired on 22 December 1999 (a), and 25September 2002 (b). The cofferdams were noticeable in 1999 and they have been removed in2002.

The Three Gorges Reservoir, China 4003

Figure 2. The study area outlined over the SRTM DEM data.

Figure 3. The missing elevation data or voids of the SRTM DEMs are shown in black. Thearrow points to the dam site.

4004 Y. Wang et al.

causing the mountain front to be foreshortened or laid over (e.g. Lillesand et al.

2004). It is impossible to define the elevations if that happens (personal email

communication, Tom Farr at the Jet Propulsion Laboratory, 2004). The layover

occurs in many locations in the study area, especially within the gorges due to the

steep slopes of the mountains.

2.3 The removal of the voids

To remove the voids, two methods are used in order. First, a search and replacementof individual voids within a 363 window is conducted. If and only if the central

pixel is a void, then its elevation value (z) is replaced by

zvoid~IntSum of the elevation values of the surrounding 8 pixels

8z0:5

� �ð1Þ

Int is the integer operation. Thus, the replaced value is still a signed 16-bit integer. It

should be noted that this approach comes from the typical method that replaces

random bad pixels or shot noise (e.g. Jensen 2005). After the operation from the first(upper left) pixel of line 1 to the last (lower right) pixel of the last line, all individual

voids surrounded by cells of valid elevation data are removed and replaced. If there

are two or more voids together locally, occurring within the study area, the second

method will be carried out.

Since the rivers in the gorges are typically tributaries of Changjiang, the surface

water heights of the streams/rivers within the gorges should be very close to the

surface water height of Changjiang. (We have confined the study area, e.g. figure 2,to avoid places such as waterfalls where large change of stream/river surface water

heights could occur.) Thus, the use of the elevations of surface water heights of

Changjiang to replace the voids is chosen. (Other possible ways to find-and-replace

the voids will be discussed later.) To create the elevation profiles, the river gauge

readings of surface water heights along Changjiang within the study area are needed.

Based on the river gauge data available to the general public (Tang 1990, Lin 1992),

the lowest and highest annual surface water heights of the Changjiang at Chongqing

City gauge station are 159.5 and 192.8 m above the mean sea level, respectively.Its annual average is 165.7 m. At Yichang City, where Gezhou Dam (on the

Changjiang) is located, the height is 66.0 m on the upstream side of the Gezhou

Dam. We assume that the height is 66.0 m annually (or a constant) at Yichang. Also,

along the river the distances between the two cities are about 648 km. If the surface

water height of the river is assumed to decrease linearly along the river, then based

on the annual mean (165.7 m) at Chongqing and 66.0 m at Yichang, the profile of

the water surface heights of the Changjiang is obtained (figure 4). The slope of the

line is 15 cm km21. Thus, if there are multiple voids together, called void patches forshort, on the river channel, then they will be replaced with the values according to

the height profiles at that location. To remove and replace the void patches in off-

river areas, an elevation surface based on the profile is needed to correct them. To

create the surface, the same interpolated river elevation at a given location is applied

to the entire column (in north and south direction) of the surface at that location.

The simplification works for four major reasons. First, the river is mainly confined

between mountains, so that the study area delineated on both sides of the banks is

mostly a relatively narrow strip (e.g. figure 2). Second, the river generally flows fromthe west to east. The flow pattern helps exclude the situations that there are multiple

elevation values of the river at each column. The river should have multiple

The Three Gorges Reservoir, China 4005

elevation values if oxbows exist. Third, if there are oxbows, as long as the sizes of the

oxbows are not in the tens of kilometres, the oxbows should have little or no impact

on the interpolated surface. The reason is that the minimum increment of the

elevation of the DEM is 1 m, the slope of the height profiles is 15 cm/km, and no

oxbows with dimension greater than 5 km are observed within the study area (based

on the Landsat 7 and DEM data). Finally, the elevation value of the surface is only

used when there are void patches at that location. Figure 5 in the grey scale shows

the interpolated elevation surface. A black line-segment pointed by a white arrow is

shown as an example of a column of cells that have the same elevation values. Also,

from the west (left) to east (right), the elevation decreases along the river channel. In

summary, to replace all the voids of the DEM data within the study area, we first

remove individual voids using equation (1) to create intermediate DEM data. Then,

we overlay the intermediate DEM layer and interpolated elevation surface layer

(figure 5) to create the final void-free DEMs. That is, if void patches are found on

the DEMs, the elevation values from the elevation surface at the corresponding

locations are output to the final DEMs, and otherwise, the elevation of the

intermediate DEMs is output. (It should be noted that the second void-removal

method alone can be used to replace the individual voids. However, the authors

believe that the combined use of both methods should produce the DEM that is of

minimal alteration on elevation value; the elevation is one of the most critical data

for this study.) Then, the following analyses are carried out.

2.4 The water volume and areal extents of the reservoir

To compute the water volume of the reservoir at a given water surface level, we first

assume that the water surface within the reservoir is flat (e.g. figure 4). Then, the

Figure 4. The profile of surface water heights of Changjiang between the cities ofChongqing (at the 100 km mark) and Yichang (at the 748 km mark). The distancebetween the two cities is 648 km. The shaded area indicates the water volume of the reservoirat 135 m.

4006 Y. Wang et al.

DEM is inundated at that given surface level (within the study area). If the elevation

of a pixel is less than or equal to the level, that cell will be a part of the reservoir.

Otherwise, the cell will not belong to the reservoir. Next, for each cell within the

reservoir, a height difference between the DEM value and water level is calculated.

The difference is further multiplied by the cell size to compute the volume (of water)

in that cell location or water column. By summing all the water columns, we obtain

the total volume that is made of all the water added to the reservoir area after the

construction of the dam. Thus, the volume is not only a function of the water level

within the reservoir, but also the river’s surface water height (or the reference) before

the completion of the dam. In this study, the river’s surface height is defined by the

DEMs after void-removal processes. (It should be noted that the flow of Changjiang

varies annually. If the SRTM flight occurred in another time of the year, the DEMs

would differ. Thus, the results of river surface area, inundated upland area, reservoir

volume, surface area, and total length could be different.) The shaded triangle in

figure 4 illustrates the volume when the water level within the reservoir is 135 m and

the DEM is used. Five reservoir water levels at 135, 145, 155, 165, and 175 m are

employed in the computation. (When completed in 2009, the designed water level

within the reservoir will vary between 135 and 175 m.)

For a given reservoir’s water level, the length of the reservoir is the distance

between the dam and end of the reservoir traced along Changjiang. The length is

obtained using head-on digitizing of the inundated DEMs on the screen. Five

heights as listed above are used.

To determine the total extent of the reservoir, one can extract all cells within the

study area where the DEM elevation is less than or equal to a given reservoir water

surface level. By summing the area of the extracted cells, the total area of the

reservoir is computed. The inundated, flat area is computed by excluding the river’s

surface area from the total extent of the reservoir. Using the slope data derived from

the DEMs, we can compute the flooded slant area as well. (It is feasible that areas of

Figure 5. Interpolated surface of the water level based on the annual mean values atChongqing (upstream) and Yichang (downstream). Each cell in a column (in north–southdirection) has the same elevation value that is equal to the interpolated water surface height ofChangjiang at that location. A black line-segment pointed by a white arrow in the middle ofthe figure shows one column, as an example.

The Three Gorges Reservoir, China 4007

an island or islands within the reservoir are not included in the area calculation as a

part of the reservoir.)

3. Results

When the water level increases from 135 to 175 m, the reservoir’s volumes increase

from 15.7 to 45.9 billion m3, the lengths measured from the dam site and along the

original river channel range from 425.2 to 672.9 km, and the total surface areas are

between 514.0 and 1077.3 km2 (table 1). The original river’s surface areas are also

given in parentheses (table 1). By subtracting the river’s areas from the total areas,

one can derive the inundated flat areas (table 2). Figure 6 shows, near Chongqing

City, (a) the total flat area in white colour, (b) the original river’s area in black

colour, and (c) the inundated flat area in white colour, as an example. The black

spots surrounded by the water area (figure 6(a)) are the islands whose areas are not

included in the total area calculation. Based on the slope of the surface derived from

the DEMs, the slant areas are obtained (table 2). The areas range from 140.8 to

555.5 km2.

4. Discussion

The method to compute the volume of water and areal extents at different water

levels within a reservoir is straightforward once the DEMs are available and the

delineation of the reservoir region is done. However, due to the existence of voids or

no elevation values within the SRTM DEMs, the search and creation of a method

that can find-and-replace the voids with the minimal alteration of the DEMs are not

so simple. A two-step preprocessing method has been discussed and developed. To

have a better understanding of why the method has been implemented, the authors

provide the following discussions.

As presented previously, individual voids and void patches occur on some of the

surface of Changjiang, in the radar shadow areas, etc. Initially, a modified version of

Table 1. The water volume, length, and surface area of the reservoir at five water levels. Theoriginal river’s surface areas are given in parentheses.

Water level (m) at Volume (billion m3) Length (km) Surface area (km2)

135 15.7 425.2 514.0 (378.0)145 21.2 476.9 617.9 (417.2)155 27.9 540.6 748.1 (462.0)165 36.1 612.8 903.0 (506.6)175 45.9 672.9 1077.3 (538.9)

Table 2. The inundated flat and slant areas of the reservoir at five water levels.

Water level (m) at Flat area (km2) Slant area (km2)

135 136.0 140.8145 200.7 207.7155 286.1 295.8165 396.4 409.4175 538.4 555.5

4008 Y. Wang et al.

equation (1) is suggested. The modifications include the increase of window size (e.g.from 363 to 565, 767, etc.), and change the rule that if and only if the central

pixel is a void. The new rule could be if and only if the number of voids is less than

half of the total number of pixels within the moving window then all elevation values

of voids will be replaced by the averaged elevation values of the non-void pixels.

However, two concerns overwhelmingly persuade the authors to give up the

modification idea. First, the selection of window size is determined by the size of the

void patches. The larger the patches in size, the larger the window size. Since each

pixel is about 90 m690 m, a 565 window, for instance, means that an area of450 m6450 m is used in the void-removal process. The dimension (length or width)

of some gorges or valleys may not be as big as 450 m. The other concern is that

Figure 6. Near Chongqing City: (a) the total flat area in white colour, (b) the original river’ssurface area in black colour, and (c) the inundated flat area in white colour. The water level ofthe reservoir is 175 m.

The Three Gorges Reservoir, China 4009

procedure itself is a trial-and-error one. Multiple iterations (starting initially with a

363 window size and then up) are needed and the modification of the original

DEMs can be too much (after several iterations).

Another possible way to find-and-replace the voids may involve the use of the

elevation value of the GTOPO30 DEMs since they have no voids. However, five

concerns make us move away from using it. The first concern is the large pixel size of

,900 m6900 m in the study area of the GTOPO30 DEMs. In some locations, the

Changjiang is even not connected on the GTOPO30 DEMs due to its narrow width

within gorges. The second concern is the great uncertainty to geo-reference the two

sets of DEMs. Even though both DEMs come with geographic coordinates, the

errors of the coordinates are so large that one could not use their coordinates

directly without performing the DEM-to-DEM registration or DEM-to-map

rectification. For example, when two sets of DEMs are overlaid directly, the

Changjiang (shown as a dark and curved feature on both DEMs) is not close to each

other. In some cases, the Changjiang is far apart, and other cases, it crosses each

other. Third, the interpolations on the (x, y, z) values of the GTOPO30 and SRTM

DEMs cannot be voided in geo-referencing. The interpolation can definitely

change the DEMs. The accuracy of the GTOPO30 DEMs is the fourth concern.

Finally, before the find-and-replace operation for the voids, the GTOPO30 DEM

should be further magnified by a factor of 10 to match the resolution of the SRTM

DEMs.

Another line-by-line find-and-replacement procedure is also sought. For each line

of data, starting from the west (or left) and moving to the east (or right), or moving

from the north (or up) to south (or down), all the elevation values of the voids

between two valid elevation pixels are replaced by the averaged elevation value of

the valid pixels. This west-to-east (or north-to-south) line method should be mostly

suitable for void patches occurring in the gorges that are parallel (or perpendicular)

to the shuttle flight direction, and the gorges are in radar shadows (caused by the

mountains). The problem, however, is that the elevation values of the voids (on the

Changjiang surface or within the gorges) can be potentially ‘elevated’ too much

because the valid pixels tend to be located on the banks whose elevations are

typically higher than those on the surface of Changjiang or in the gorges. In a worst

case where a gorge is narrow and mountains on both sides are tall and have steep

slopes, one valid pixel could be located near or on the top of the mountain on one

side, and the other near or on the top of the mountain on the other side. This

method is also discarded.

5. Concluding remarks

Details of the methodologies, assumptions, and datasets have been presented to

illustrate the use of the SRTM DEM data in the computation of the water volume,

length, total area, and inundated flat and slant areas of the Three Gorges Reservoir

of Changjiang, Chongqing Municipality, China. Due to the existence of voids or no

elevation values on the DEMs, a two-step process to find-and-replace the voids has

been carried out. Giving the assumption that the water surface within the reservoir is

flat, the total volumes of water, lengths, total areas, inundated flat and slant areas of

the reservoir are derived at the reservoir’s water levels between 135 and 175 m above

the mean sea level.

The attempt to compare the derived values with the available published data ends

without a fruitful result. There is no doubt that there are many detailed reports

4010 Y. Wang et al.

about the Three Gorges Dam project and facts about the reservoir. The issue is the

unavailability of the reports. Limited factors published at the web site of the Chinese

Embassy in the USA (last accessed in February 2005) are: at the water level of

175 m, the storage is 39.3 billion m3 (http://www.china-embassy.org/eng/zt/sxgc/

t36499.htm), the length of the reservoir is 663 km, the reservoir covers an area of

1045 km2, and the inundated slant area is 632 km2 (http://www.china-embassy.org/

eng/zt/sxgc/t36512.htm). If the official data are compared with those in this study,

there is an overall general agreement (tables 1 and 2). However, because of the lack

of details about the methods, assumptions if any, and datasets used, how the official

data were obtained is unknown. Also, it should be cautioned that the similarity in

(aggregated) values is not a warrant for one to conclude that there is an agreement

on a location-by-location basis. For example, the derived and official total surface

areas are 1077 and 1084 km2, respectively, when the water level is at 175 m. The

difference is only 7 km2 or 0.6% of the total area. However, the (small) areas making

up the total area may come from different locations. Therefore, the official values as

well as the comparison are provided for readers’ information only.

Are there any other means and/or datasets that can be used perhaps to prove or

disprove fully or partially the findings here? First, ground truthing may be

impossible due to the large areal extent and cost. Second, the storage capacity or

inundated areas below the water level of 140 m may not be re-calculated because the

reservoir started to store water in June 2003 and its current water level is ,140 m.

The SRTM DEM obtained in 2000 is the best dataset available before the water

storage occurs. Third, due to the malfunction of the scan line corrector (SLC) of

Landsat 7 that occurred in May 2003 (http://landsat7.usgs.gov/index.php, last

accessed in February 2005), the Landsat 7 data collected after the SLC failure may

not be usable because the study area is generally in the east–west orientation. The

research communities lose one data source that provides the global coverage and is

affordable, which hampers the effort of verification. Finally, the Three Gorges Dam

project is still in progress. However, one should be able to re-compute the length and

total area, and inundated flat and slant areas at water levels between 135 and 175 m

in the future. Once the reservoir is fully in operation, the water level will be

controlled to vary annually between 135 and 175 m to help prevent flooding in the

reservoir region as well as areas downstream to the dam, and to generate electricity.

Two sets of remotely sensed images covering the entire reservoir region should allow

the re-computation as long as one set of data is acquired before June 2003 and the

other set is obtained with a known reservoir water level. Because the JERS-1, ERS-

1, and Landsat 7 images that cover the reservoir region were acquired before June

2003 and are already in-house, and we will also be provided with the future radar

and optical data from the Japanese Advanced Land Observation Satellite (http://

www.jaxa.jp/missions/projects/sat/eos/alos/index_e.html, last accessed in February

2005), we should be able to use these remotely sensed data to verify the findings in

the future.

Acknowledgment

This study is supported by the 973 Program of China through the contract

(No. 2003CB415205) to the Wuhan University, China, and by the Center for

Geographic Information Science of the East Carolina University, North Carolina,

USA.

The Three Gorges Reservoir, China 4011

ReferencesADAMS, P. and RYDER, G., 1998, China’s great leap backward. International Journal,

Autumn, pp. 686–704.

BECKER, J., 1999, Despite promises, resettlement prospects look bleak for masses displaced by

Three Gorges Dam project. South China Morning Post, 14 February, Sunday Focus

p. 7.

BERGMAN, E.F. and RENWICK, W.H., 2004, Introduction to Geography: People, Places, and

Environment, 3rd edn (Upper Saddle River, NJ: Prentice Hall).

DAI, Q., 1994, Yangtze! Yangtze! Debate over the Three Gorges Project (London and Toronto:

Earthscan).

JENSEN, J.R., 2005, Introductory Digital Image Processing: A Remote Sensing Perspective, 3rd

edn (Upper Saddle River, NJ: Pearson/Prentice Hall).

LILLESAND, T.M., KIEFER, R.W. and CHIPMAN, J.W., 2004, Remote Sensing and Image

Interpretation, 5th edn (New York, NY: John Wiley).

LIN, B., 1992, Engineering Sediments (Gong Cheng Ni Sha in Chinese) (Beijing: Press of the

Hydroelectricity) (in Chinese).

MURPHY, D., 2002, Dam the consequences. Far Eastern Economic Review, May 23, pp. 28–30.

SULLIVAN, L.R., 1995, The Three Gorges project: dammed if they do? Current

History: A Journal of Contemporary World Affairs, September, pp. 266–269.

TANG, R., 1990, Book Series of the Gezhou Dam: Sediment Engineering (Ni Sha Gong Cheng in

Chinese) (Beijing: Press of the Hydroelectricity) (in Chinese).

TIAN, F. and LIN, F. (Eds), 1989, A Second Look at the Macro-level Decision Making on Three

Gorges Project (Changsha, China: Hunan Press of Science and Technology) (in

Chinese).

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