Google於2005年推出谷歌地圖(Google Maps)後，成功將網際網路地圖服務(Web Map Services, WMS)推廣至一般使用者，成為目前一般使用者最直覺獲取空間資訊的方式。但美國國家地理空間情報署(National Geospatial-intelligence Agency, NGA)以及國際油氣生產者協會(International Association of Oil & Gas Producers, IOGP)卻相繼建議使用者勿在網際網路麥卡托投影(Web Mercator Projection)上進行重要量測工作。在網際網路地圖上量測，除了會受限於原本圖資的測製精度之外，也會受到地圖投影方式、螢幕點選精度、圖臺設計等影響，導致量測結果與實際有所出入。本研究以使用者觀點出發，採用實驗之方式，試圖釐清網際網路地圖量測準確度，以提供一般使用者參考。本研究以國內外常使用的網際網路地圖，如：Google Earth Pro、Google Map、ESRI ArcGIS Earth、ESRI ArcGIS Online、國土測繪中心臺灣通用電子地圖、國土測繪中心圖資服務雲以及國土測繪中心基本地形圖共7個網際網路地圖為實驗圖臺，分為兩個範圍進行實驗。第一部分的實驗區域以臺灣本島地區120°E、121°E、122°E三條經線及23°N、24°N、25°N三條緯線所交會出的 9個整數經緯度交點為坐標量測目標，以兩點之間的12條邊長作為距離量測標的，評估使用者在使用上述網際網路地圖服務時，所能得到之量測準確度，以及利用Google 以及ESRI所提供的API進行距離計算，了解摒除人為點選誤差後，網際網路麥卡托投影於臺灣地區的變形規律。第二部分的實驗區域以臺北市區為範圍，挑選東西方向的信義路與南北方向的松江路(新生南路)，分別進行線段兩端點的量測，釐清使用者於較小範圍內，所能得到的量測準確度。最後以信義路與松江路(新生南路)上實際的GPX軌跡檔，利用API進行距離計算，評估採用API進行軌跡距離計算之準確度，並了解利用網際網路地圖所提供的API進行距離計算是否適切。

Since Google launched its first open-access web map application, Google Maps, in 2005, the web map service (WMS) has been widely used by people all over the world. The convenience and open accessibility of the WMS breaks down the high barrier of using professional paper map. These days, people are more accustomed to measuring coordinates and distance on the WMS rather than on the paper maps now. However, the U.S. National Geospatial-Intelligence Agency (NGA) and the International Association of Oil and Gas Producers (IOGP) have successively been publishing notices dissuading people from measuring distances on the WMS, based on results produced by the Web Mercator Projection. Since distortions are inevitable for any projection, every map would therefore contain some form of distortion. Nevertheless, measuring errors occurring on the WMS derives not only from the projective distortion, but also from both the human-machine interface and the web map platform. The objective of this research is to assess the overall accuracy of the measurement on the WMS. Seven web map services are chosen for experiments: Google Earth Pro, Google Map, ESRI ArcGIS Earth, ESRI ArcGIS Online, NLSC Taiwan Electronic Map, NLSC Basic Topographic Map, and NLSC Taiwan Map Service. The experiments consists of two parts, the first part covers the whole Taiwan while the second part puts focus on Taipei City. In the first part, we chose nine intersections where three parallels (23°N, 24°N, 25°N) and three meridians (120°E, 121°E, 122°E) meet as the measuring targets. The distances of the 12 edges formed by the nine intersections are measured for accuracy assessment. The coordinates of the nine intersection points derived from the Web Mercator Projection formula are assessed for the theoretical accuracy. Next, we invited several participants to click 10 times on each of the nine intersection points to evaluate the experimental accuracy of the coordinates. Finally, the measured distance of the 12 edges are compared to the geodesic length to conclude with an evaluation of the experimental accuracy on the distance measurements. In the second part of the experiment, we choose a south-north-bound road and a west-east-bound road in downtown Taipei City, Taiwan. The west-east-bound is Xinyi Road and the south-north-bound is Songjiang Road (Xin-sheng South Road). First, the Web Mercator Projection formula is used to assess the theoretical accuracy of the coordinates of the start and end points of each road. Secondly, distance between the coordinates of the two points is measured by the user and calculated from the Google Maps JavaScript API or ArcGIS API. Thirdly, the driving route distance is calculated from the GNSS track file - GPX. Finally, all measured or calculated distances are compared to the geodesic length to evaluate its experimental accuracy. The final results of this research can be served as a reference to people who prefer to make measurements on the WMS.

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