Geodetic Data for Juneau Icefield Glaciers: Orthophotos, Digital Elevation Models, and Glacier Boundaries

Since the 1940s, the Juneau Icefield Research Program (JIRP) has been measuring surface mass balance on the Juneau Icefield. This is the longest ongoing program of its kind in North America. The program nominally occurs between late June and late August, traversing between Juneau, Alaska and Atlin, British Columbia. JIRP has examined the surface mass balance of the Juneau Icefield since 1946, with principal efforts focused on Lemon Creek Glacier and Taku Glacier. Glaciological, geodetic, and meteorological data have been collected by JIRP to characterize the interaction between the climate and glaciers of the Juneau Icefield. Direct field measurements of point glaciological data are combined with weather and geodetic data to estimate the seasonal and annual mass balance at each glacier in both a conventional and reference surface format (Cogley and others, 2011). The basic analysis framework (O'Neel, 2019, McNeil and others, 2019) is the same at each glacier to enable cross-comparison between output time series. For Taku and Lemon Creek glaciers, temperature lapse rates are optimized using on-icefield weather data. This changes the degree day factor in the melt model, giving small post-geodetic calibration differences on the order of 2-3 cm. Details are described in McNeil (2019). Vocabulary used follows Cogley and others (2011) Glossary of Glacier Mass Balance. This portion of the data release includes geodetic data used in mass balance analyses. The USGS uses geodetic data to quantify glacier area, glacier hypsometry, and the change in glacier volume and mass (e.g., Cogley and others 2011; Zemp and others 2013; van Beusekom and others, 2010; O'Neel and others 2014). Here we describe these basin-scale data, how they are produced, and the format in which they are preserved and disseminated. Gridded products comprise the first class of data and include orthorectified images and Digital Elevation Models (DEMs). Prior to the early 2000s, these grids were derived from aerial stereo photography or historic topographic maps. More recently, high-resolution space-borne imagery facilitated DEM and ortho-image production using approaches described herein. The second class of data are vector geospatial files of glacier margins. These are interpreted products, produced via manual digitization of the boundary between rock and ice identified from ortho-rectified images, or the boundary between neighboring glaciers identified from ice divide velocity maps. Additional data for Lemon Creek Glacier, part of the Juneau Icefield, is available in a separate data release of USGS Benchmark Glacier geodetic data at https://doi.org/10.5066/P9R8BP3K. The data is documented but is not included here in order to avoid duplication.

自20世纪40年代起，朱诺冰原研究项目（Juneau Icefield Research Program, JIRP）便持续对朱诺冰原的地表物质平衡开展观测，该项目是北美地区同类研究中持续时长最长的一项。项目常规执行周期为6月末至8月末，考察路线横跨阿拉斯加州朱诺市与不列颠哥伦比亚省阿特林市之间的区域。JIRP自1946年起便聚焦朱诺冰原的地表物质平衡研究，核心观测重点为勒莫恩溪冰川（Lemon Creek Glacier）与塔库冰川（Taku Glacier）。
该项目收集了冰川学、大地测量学与气象学相关数据，以解析朱诺冰原区域气候与冰川间的相互作用。研究人员将点位冰川学实地直接测量数据与气象、大地测量数据相结合，参照Cogley等人（2011）的标准，采用常规与参考地表两种格式，估算各冰川的季节与年度物质平衡。所有冰川均采用统一的基础分析框架（O'Neel，2019；McNeil等人，2019），以实现不同冰川产出时间序列数据的跨项目对比。针对塔库冰川与勒莫恩溪冰川，研究人员利用冰原原位气象数据优化了温度递减率，该操作调整了融冰模型中的度日因子，使得大地测量后校准结果产生约2至3厘米的微小差异，相关细节详见McNeil（2019）的研究内容。本数据集所采用的专业术语遵循Cogley等人（2011）发布的《冰川物质平衡术语表》。
本次数据发布包含用于物质平衡分析的大地测量数据。美国地质调查局（USGS）借助大地测量数据量化冰川面积、冰川面积-高程分布、冰川体积与质量的变化（例如Cogley等人，2011；Zemp等人，2013；van Beusekom等人，2010；O'Neel等人，2014）。本文将对这些流域尺度的数据集、其生成方式，以及数据的存储与分发格式进行详细说明。
第一类数据为网格化产品，包含正射校正影像与数字高程模型（Digital Elevation Models, DEMs）。2000年代早期之前，这类网格化数据均通过航空立体摄影或历史地形地形图生成。近年来，高分辨率星载影像推动了DEM与正射影像的生产，所用具体方法详见本文所述内容。第二类数据为冰川边界矢量地理空间文件，这类数据属于解译产物，通过对正射校正影像中岩冰边界进行手动数字化，或通过冰分水岭速度图识别相邻冰川间的边界而生成。
作为朱诺冰原组成部分的勒莫恩溪冰川的额外数据集，可通过美国地质调查局基准冰川大地测量数据的独立发布页面获取，链接为https://doi.org/10.5066/P9R8BP3K。为避免数据重复发布，本次数据发布未包含该部分内容，但相关文档已完整齐备。