數值高程模型(Digital Elevation Model, DEM)是以數值方式來儲存、表現地表的起伏，應用的層面相當廣泛，為土地利用的重要資訊。近年來測量技術的進步，除了傳統的航測的人工量測外，數值影像匹配、空載光達和干涉合成孔徑雷達的發展，可以利用不同的技術快速的得到DEM。然而不同方式製作的DEM，可能造成的誤差都不同。因此除了製作過程中的品質檢核外，評估已完成的DEM資料品質，亦為一重要的課題。
測量數據的品質包含了精度(precision)、精確度(accuracy)、可靠度(reliability)三方面的驗證。一般DEM 品質評估指標的方式為採用抽樣檢測高程的精度，計算均方根誤差(Root Mean Square Error, RMSE)值。但是此方式並不能充分的描述資料品質，因為此種評估方式無法提供空間中誤差的分佈和極值的資訊。而本研究對於DEM的品質評估則是採用兩個方法：1.內部評估2.相對的外部評估。內部評估主要是根據DEM的原始資料點，利用最小二乘內插原理，以一個平面來擬合估算每個網格點內插後的高程精度。而相對的外部評估則是將不同時期不同來源的DEM 相互比較，檢視彼此間的高程差值，以判斷是否有系統誤差存在，探討誤差的合理性和邏輯性。
本研究實驗區為南化水庫，分別測試了航測和光達的資料，實驗結果證明內部精度的評估可以在無外在獨立資料情況下，有效的估計DEM的精度。外部精度的評估則是比較了四種DEM，由結果發現，DEM受到地形及覆蓋物的影響，即使是高精度的光達1公尺和航測5公尺解析度的DEM，在山區的相對誤差有時仍會達到20公尺不合理的差距。從比較中也可看出不同方式製作的DEM 所產生的誤差，例如光達點雲的過濾使山脊部份過度平滑、航測DEM 錯估植被覆蓋的高度等誤差問題。此結果可提供給DEM使用者一個參考。

DEM has long been important land-use information to many applications because it records height information of terrain surface. With the great progress of measuring technology, DEM could be acquired quickly and efficiently via digital photogrammetry, airborne LiDAR and InSAR. However, the procedure from raw data sources to finale DEM products may involve different types of errors, which would influence the final DEM quality more. For this reason, demand for quality control and assessment arises, then to provide effective and objective quality assessment for DEMs becomes an important topic.

The quality of measuring data is generally assessed in terms of precision, accuracy, and reliability. It is generally adopted to assess the DEM quality by the index of RMSE. The RMSE value gives whole accuracy index of the DEM quality, but lack to describe interior local variances and relative comparison with other DEM data. So this study proposed two methods to describe DEM quality—interior and exterior relative quality assessment. Instead of whole index of RMSE, the former provides accuracy index of each height point calculated from the raw data to reveal the DEM quality of local. Then the later explores the relative errors between DEMs acquired from different sources and times. The two methods provides the assessment of the internal quality and exterior relative accuracy index for DEMs respectively, which is hoped to give more complete and objective quality information.

DEMs generated from airborne LiDAR, photogrammetry, and InSAR are employed to test feasible of proposed methods. The main test area is at Nan-Hua Reservoir, Tainan. The interior assessment shows the DEM quality relates to the terrain type, distribution and density of sampling points. It helps to realize the DEM quality of local details. Then comparing present domestic DEMs, the relative assessment finds DEMs of different data sources often make relative big errors. For example, the visible errors between DEMs of LiDAR and photogrammetry usually occur in rough terrain or mountain ridges, where the maximum error ups to 20 m. Through a serious of experiment tests, it is proven that the two methods provide useful information to DEM quality assessment, which makes a user to realize the quality of acquired DEMs in detail.

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