The Hong Kong 7/1 March: Crowd Size Estimates (Ming Pao Aerial Photo Analysis)



(Ming Pao)  Computer analyses showed 264,000 marchers

[Translated summary]  Was it 200,000?  530,000?  Every time there is a large-scale event, the number of marchers is a disputed figure.  We asked a satellite photo analysis specialist to examine the photos that we took from a helicopter.  The final estimate is 264,000.  

According to our expert Thomas Lee, in the Victoria Park soccer fields, there were about 100 persons for every 100 square meters.  Based upon a road surface of 72,000 square meters (=3,600 meters long and 20 meters wide), there were about 72,000 persons at 4pm.  Under the assumption of a travel time of 90 minutes, there should be a total of 3.67 groups in the 5.5 hour time span.  Therefore, the total estimated crowd size is 3.67 x 72,000 = 264,000 persons ignoring those who joined in the middle.  The margin of error is 15%.

According to Civil Human Rights Front spokesperson Jackie Hung, the police estimated that the route capacity is 170,000 persons whereas the six soccer fields in Victoria Park contains 110,000 persons.  Hung does not believe that the route can hold only 70,000 persons, because there has to be more on the road than inside Victoria Park.


從事衛星圖片科技研究,身兼香港大學地理系遙感課程(remote sensing)及理工大學衛星定位(GPS)應用課程的客席講師的星眺有限公司負責人李偉鵬,昨日透過一種名為「太空科技衛星圖片分析軟件」(image remote sensing software),用多光譜分析原理,配合人體的大小及形狀,從空中圖片中找出「一個個的人型物體」作評估,然後統計特定空間內的人群數量。







This project in fact consisted of two phases.  In the first phase, Ming Pao hired a helicopter and took aerial photos.  These photos were then submitted to the satellite photo analysis expert Thomas Lee, who used his specialized software to count the number of people.  Thomas Lee is a honorary and visiting lecturer at Hong Kong University (Remote Sensing and Photogrammetry) and Hong Kong Polytechnic University (Global Positioning System in Logistics), where he teaches classes on how to use remote sensing software to count tree density, identify land use, position objects, etc.

Although many photos were taken, not all were suitable for use.  For example, it would be difficult to discern people after it got dark; or when the buildings cast deep shadows on the marchers.  So the best photos were those that were taken while the marchers assembled on the open soccer fields in Victoria Park.  On the basis of those photos, Thomas Lee estimated that the average density was 100 persons per 100 square meters.  He also noted that he counted sometimes as few as 85 persons per 100 square meters in some photos but other times as many as 115 persons per 100 square meters elsewhere.  This is not a statement of the margin of errors as such, but the natural variation in the data distribution itself.  Thomas Lee actually recommended that the less dense factor of 85 per 100 square meters ought to be use.  

Thomas Lee also used accessed a digital map database and geographical information system (GIS) to study the march route.  He determined that the total length of the route was 3,600 meters.  The width of the road was 20 meters on averages, although it could be more or less in some places.  In the end, he used the 20 meter figure to obtain an estimate of the total area being 20m x 3,600m = 72,000 square meters.  In addition, he also recommended adjustments to be made for people leaving before the end or joining after the start. 

This was the information that Thomas Lee provided to Ming Pao.

In the second phase, Ming Pao took that information and made their own estimate of the march size.  In so doing, they made one major error which eventually required a retraction and some other possibly erroneous assumptions.

First of all, Ming Pao decided to use the density of 100 persons per 100 square meters, in spite of the Lee's recommendation of 85 meters.  This resulted in the estimate of 72,000 persons per trip.

The major error was the same "tree planting problem" error made by the Civil Human Rights Front.  Ming Pao had made the non-physical assumption that the number of trips was 5.5 / 1.5 = 3.67.  It is now accepted that the bulk of the marchers left between 230pm and 545pm, so that there is only time for 2.17 trips.  At a density of 72,000, the number of marchers is 72,000 x 2.17 = 156,000.  I emphasize again this was Ming Pao's error and it was not the fault of Thomas Lee. 

In addition, Ming Pao assumed that the density of stationary people in Victoria Park would be the same as when they get on the road.  This is surely wrong.  I can stand right next to someone, but when I walk, I would need some more space to avoid collision or tripping unless we all walk very slowly.  Empirically, you can observe this phenomenon when you disembark from Star Ferry: the passengers are crowded together as the ferry approaches the pier, but they space out as soon as they start walking on land.  Alternately, you can compare a full parking lot against vehicular traffic in motion.  So the density appears to be overstated for the march.

Finally, the density of the stragglers at the tail end of the march is likely to be less than the front end.  By 6pm, the march route had been closed from 6 lanes down to 3 lanes in Wanchai and the same in Admiralty by 7pm.  Ming Pao had assumed a constant density, and this is another overstatement.

(Oriental Daily)  Satellite Photo Accurately Calculates The Number of Marchers  July 3, 2004.

[translation] The police and the Civil Human Rights Front had a large difference in estimating the number of marchers.  According to a satellite photo analysis expert who used a large-scale analysis to estimate the number of marchers on the basis of the road area, the total road area is about 72,000 square meters.  If each 100 square meters contain 85 to 100 persons, then the whole roadway would contain 72,000 if filled.  Therefore, he estimated that the number of marchers would be 220,000, which is about the same as the police estimate.

Lee Wei-peng is the general manager of a company that does land surveying, and he used an aerial photograph taken by a newspaper and applied a piece of software that is used for satellite photo analysis of large-scale geographical/environmental research and used the principles of diffraction to estimate the number of marchers.  He pointed out that the density of the crowd in the photo varied from sparse to dense.  Each 100 square meters contain 85 to 100 persons.  When a Victoria Park soccer field is filled, there are about 20,000 persons.

He said that the march route went from Victoria Park to Battery Lane in Central.  Subtracting the tram lanes that the police set aside for emergency vehicles, the entire road way is about 3,600 meters long and 20 meters wide.  The total area is 72,000 square meters.  If the entire roadway is filled with stationary people, without considering people who might have left or joined the march in the middle, the total number of persons on the roadway is 72,000.

He pointed out that the marchers must move forward, which would lead to spaces appearing in the road.  To accurate estimate the number of marchers, it would be necessary to take multiple photos from the start to the finish, including the starting and end points, in order to determine the march time and the number of waves of people.  If it is assumed that there were three waves during the period, then the number of marchers is estimated to be 220,000 which is about the same as the police estimate.

He pointed out that aerial photo analysis of march sizes is seldom done, but it has known accuracy.  He estimates that the cost for this estimation project was between $50,000 to $100,000.

He believes that the police error is unlikely to be subject to a lot of error.  But he thinks that the Civil Human Rights Front may have included those who left or joined the march in the middle, so it may not be surprising that they differ by twice as much.  He suggests that the two parties should describe their methods and compare.

衛星圖片準確計算遊行人數 新 聞

警 方 及 主 辦 七 一 遊 行 的 民 間 人 權 陣 線 , 對 遊 行 人 數 統 計 存 在 極 大 差 異 , 有 星 圖 片 分 析 專 家 以 專 門 為 大 型 規 劃 分 析 的 方 法 , 以 路 面 面 積 計 算 遊 行 人 數 , 他 指 整 條 遊 行 路 線 的 面 積 約 七 萬 二 千 平 方 米 , 每 平 方 米 約 八 十 五 至 一 百 人 計 算 , 填 滿 整 條 路 線 約 有 七 萬 二 千 人 , 估 計 遊 行 人 數 約 有 二 十 二 萬 人 , 與 警 方 估 計 相 若 。

本 身 是 土 地 測 量 師 的 星 眺 有 限 公 司 董 事 總 經 理 李 偉 鵬 , 日 前 利 用 報 章 一 幅 於 維 園 高 空 拍 攝 的 圖 片 , 以 專 門 作 大 型 規 劃 、 地 質 研 究 或 空 氣 污 染 的 星 圖 片 分 析 軟 件 , 用 多 光 譜 分 析 原 理 評 估 遊 行 人 數 。 他 指 由 於 圖 片 中 人 群 有 疏 有 密 , 每 平 方 米 約 有 八 十 五 至 一 百 人 計 算 , 填 滿 六 個 維 園 足 球 場 約 有 二 萬 人 。

他 表 示 , 整 條 遊 行 路 線 以 維 園 至 中 環 炮 台 里 , 扣 除 警 方 畫 為 緊 急 通 道 的 電 車 路 段 , 全 段 路 面 長 約 三 千 六 百 米 , 闊 約 二 十 米 , 整 體 面 積 約 七 萬 二 千 平 方 米 , 若 整 段 路 面 填 滿 不 動 的 人 群 , 未 計 中 途 插 隊 或 離 隊 人 士 , 最 多 有 七 萬 二 千 人 。

他 指 , 當 遊 行 人 士 向 前 移 動 時 , 便 會 有 位 置 出 現 疏 落 情 況 , 要 準 確 計 算 遊 行 人 數 , 必 須 從 高 空 以 垂 直 拍 攝 方 式 , 由 起 點 及 終 點 每 個 段 落 拍 攝 , 再 以 確 實 行 畢 的 遊 行 時 間 及 出 發 次 數 , 再 減 除 圖 片 中 的 空 隙 , 便 可 準 確 計 算 遊 行 人 數 。 以 全 個 遊 行 時 間 共 有 逾 三 程 計 算 , 推 算 遊 行 人 數 約 有 二 十 二 萬 人 , 與 警 方 估 計 相 若 。

他 指 以 星 圖 片 分 析 軟 件 統 計 遊 行 人 數 並 不 常 用 , 但 有 一 定 的 準 確 性 , 以 今 次 大 型 遊 行 計 算 估 計 只 需 花 費 五 至 十 萬 元 。

他 相 信 警 方 的 數 字 偏 差 度 不 大 , 但 認 為 民 陣 可 能 有 計 算 離 隊 及 插 隊 人 數 , 統 計 數 字 與 警 方 相 差 一 倍 亦 不 奇 , 建 議 雙 方 應 列 出 計 算 方 式 作 比 較 。

(Ming Pao)  Yip Siu-fei: 192,000 marchers by computer analysis.  July 16, 2004.

[translation]  The author and the team he led used two counting stations and a sample survey to estimate the number of 7/1 marchers.  We were looking for a more effective and accurate method of estimating the number of marchers.  The author's method consists of counting the flow within every minute out of 15 minutes, and then used that as the average of the 15 minutes from which we derived the total number of marchers.  At the second counting station, we also interviewed a sample of people in order to estimate the number of people who joined in the middle.  We added another 10% for those who joined the march by other means.  After adjustment, the total number of marchers was estimated to be 165,000 with a confidence interval between 140,000 and 190,000.

Ming Pao used a helicopter to take photographs which were analyzed by a satellite photo expert by computer software.  This is an important point of reference.  It was estimated that there were 72,000 persons on the road between Victoria Park and Admiralty.  According to Ming Pao, the total march lasted 5 hours and 30 minutes.  Since the trip took 90 minutes, there were 3.67 waves in total and the number of marchers was 264,000.

But like the Civil Human Rights Front, Ming Pao made the same error for the "tree planting" problem and double-counted the last trip.  If we began counting by the time that the first wave reached the Central Government Office, there was only 4 hours  If the trip took 90 minutes, then there were only 2.67 waves and not 3.67 waves.  The actually number of marchers is 192,000, and not the 264,000 that was reported.  Furthermore, based upon the author's observation, the 72,000 figure was obtained at the peak time (4:30pm).  After 6pm that day, the march route was reduced from 6 lanes to 3 lanes.  After adjusting for this, the number from the aerial photo analysis should be less than 192,000.

Since some media have commented about the discrepancy between the 164,000 from the author and his team and the 264,000 from the aerial photo analysis as compared to the 530,000 from the Civil Human Rights Front, the author believes that your newspaper should issue a correction.  All other newspapers and estimating organizations and individuals have published numbers that are less than 200,000, which is consistent with the police estimate.  We have reason to believe that the number of marchers is not more than 200,000 is supported by data.  The estimate from the Ming Pao aerial photo analysis is quite close to the estimate obtained by the author with human counting.





(Ming Pao)  Sorry, We Got The Number Of Marchers Wrong!  July 15, 2004.

[translation]  IT is two weeks since Hong Kong saw the July 1 march, but controversy persists over the number of people who joined it. It should fall between 200,000 (the police's estimate) and 530,000 (the figure provided by the Civil Human Rights Front, the organiser). It was reported yesterday in the press that Dr John Bacon—Shone, a Hong Kong University academic, puts it between 105,000 and 120,000.

The same day the present writer read Hong Kong University statistics and actuarial science senior lecturer Dr Paul Yip's contribution to this newspaper's Forum Page. It says the story titled "264,000 March According to Computer Analysis" which appeared on page A11 in the July 2 issue of Ming Pao contains a serious error. It says the number of the last "batch" of protesters was twice included. We must not only thank Dr Yip for pointing out this error but also set the record straight and apologise to our readers.

The long and short of the matter is as follows. We at Ming Pao consider the number of people taking part in a march to be quite newsworthy. However, over the years, the police's estimates have been at variance with and often vastly different from march organisers'. For example, the figure which the organiser of this year's July 1 march gave is more than 100% higher than the police's estimate.

To arrive at the truth we made new attempts. This year we rented a helicopter, from which our people took photographs of the procession right above it. We then asked Thomas Lee, a satellite photo analyst, to count marchers on the route between Victoria Park and the Central Government Offices (those in a "batch") using space technology — a piece of software for analysing satellite photos. He estimated that there were about 72,000 marchers in a "batch".

It took 90 minutes for protesters to march from Victoria Park to the Central Government Offices. The march lasted 5.5 hours. We reckoned there were 3.67 "batches". Therefore, 26,4000 people marched on July 1. However, there we made a "tree planting problem" error. We included the last "batch" twice. Our figure is 72,000 bigger than the true number, which should be 92,000.

Apart from Dr Yip, quite a few academics from Hong Kong University, the Chinese University, the University of Science and Technology and Baptist University have done much research to estimate the number of people who marched on July 1. By making such efforts, they have brought into play the civic spirit and the scientific spirit. They have provided the public with estimates that are relatively scientific and accurate. Those academics have employed different methods, which have their peculiar strengths and weaknesses. However, they have largely obtained relatively accurate estimates by scientific means. They have put the number within reasonably narrow neighbourhoods so that we have what is close to the truth. We ought to salute them.

The number of people who marched on July 1 this year matters. Of that the government's withdrawal of the Article 23 bill in the wake of the July 1 march last year is corroborating evidence. However, credibility matters more. It is wrong to overestimate or underestimate such numbers for any political reasons.

Fewer people marched on July 1 this year than last year. Still, that day about 100,000 citizens took to the streets, sweltering and soaked with sweat. They did express their dissatisfaction with the government and their aspirations for democracy loud and clear. Only by heeding the people's wishes, increasing democracy and improving the SAR's governance can the government allay public resentment.

March organisers' estimated numbers of participants do not seem objective, nor do the government's or the police's. That may unnecessarily arouse controversy. When organisers or the police make public their estimates, they should also disclose the methods they use and the sources of their data so that professionals, academics and citizens can form their own judgements.

We hope academics will persist in fulfilling intellectuals' functions and vigorously helping foster the civic spirit in Hong Kong. Their selfless commitment does underscore Hong Kong's strengths as a mature civil society and epitomise professionalism and the core values on which Hong Kong prides itself.











This official Ming Pao correction now assumes that the density of the crowd was constant between the departure times of 230pm and 630pm from Victoria Park (and corresponding arrival times of 400pm and 800pm).  It has now been accepted that the last significant departures had occurred by 545pm, so that the corrected figure is still an overstatement.  If you accept the 545pm time as the last departure, then the corrected figure is 2.17 x 72,000 = 156,000.

The above is a minor quibble.  Since Ming Pao disclosed what its assumptions and calculations were, the reader can make his/her own evaluation.  In any case, the behavior of Ming Pao has been honorable and exemplary to this point.