USA Legacy NLCD Impervious Surface Time Series

Impervious surfaces are surfaces that do not allow water to pass through. Examples of these surfaces include highways, parking lots, rooftops, and airport runways. Instead of allowing rain to pass into the soil, impervious surfaces cause water to collect at the surface, then run off. An increase in impervious surface area causes an increase of water volume which needs to be managed by stormwater systems. With the flow come pollutants, which collect on impervious surfaces then discharge with the runoff into streams and the ocean. Runoff water does not enter the water table, and that can cause other management issues, such as interruptions in baseline stream flow. The NLCD imperviousness layer represents urban impervious surfaces as a percentage of developed surface over every 30-meter pixel in the United States. Dataset Summary Phenomenon Mapped: The proportion of the landscape that is impervious to water. Time Extent: 2001, 2004, 2006, 2008, 2011, 2013, 2016, 2019, and 2021 for the lower 48 conterminous US states. A small portion of Alaska around Anchorage displays a time series of 2001, 2011, and 2016. Hawaii, Puerto Rico, and the US Virgin Islands unfortunately only have data for 2001 so there is only one image in the series. This information may be used in conjunction with the USA NLCD Land Cover layer. Units: Percent Cell Size: 30 meters Source Type: Discrete Pixel Type: Unsigned integer Data Coordinate System: North America Albers Equal Area Conic (102008) Mosaic Projection: North America Albers Equal Area Conic (102008) Extent: CONUS, Hawaii, A portion of Alaska around Anchorage, District of Columbia, Puerto Rico NoData Value: 127 Source: Multi-Resolution Land Characteristics Consortium Publication Date: June 30, 2023 Time Series By default, this layer will appear in your client with a time slider which allows you to play the series as an animation. The animation will advance year by year, but the layer only changes appearance every few years in the lower 48 states, in 2001, 2004, 2006, 2008, 2011, 2013, 2016, 2019, and 2021. To select just one year in the series, first turn the time series off on the time slider, then create a definition query on the layer which selects only the desired year. Time Series Descriptor MRLC issued a set of companion rasters with this impervious surface layer showing the reason why each pixel is impervious. This companion layer, called the Developed Imperviousness Descriptor, is not currently available in this map service. The descriptor layer identifies types of roads, core urban areas, and energy production sites for each impervious pixel to allow deeper analysis of developed features. The descriptor layer may be downloaded directly from MRLC and added to ArcGIS Pro. Alaska, Hawaii, and Puerto Rico At this time Alaska, Hawaii, and Puerto Rico are produced with a different methodology, and are not set up to be directly compared the way the CONUS time series is. To analyze change between the latest two data years for this portion of Alaska, be sure to use the NLCD 2011 to 2016 Developed Impervious Change raster. For Hawaii and Puerto Rico, only the year 2001 is available for download at the MRLC. North America Albers Projection All NLCD layers in the Living Atlas are projected into the North America Albers Projection before serving in the Living Atlas. This allows the coterminous USA, Puerto Rico, Hawaii, and Alaska to be served from a common projection and analyzed together. In tests performed by esri, the NLCD land cover classes after projection to North America Albers had the exact same number of pixels in input as output, but pixels had been slightly rearranged after projection. Processing Templates This layer comes with two color schemes, cool and warm. The default is a cool gray color scheme, designed to look good on light and dark gray web maps. To choose a warm color scheme which was the default until 2021, change the processing template to the Impervious Surface Warm Renderer in your map client. The National Land Cover Database products are created through a cooperative project conducted by the Multi-Resolution Land Characteristics Consortium (MRLC). The MRLC Consortium is a partnership of federal agencies, consisting of the U.S. Geological Survey, the National Oceanic and Atmospheric Administration, the U.S. Environmental Protection Agency, the U.S. Department of Agriculture, the U.S. Forest Service, the National Park Service, the U.S. Fish and Wildlife Service, the Bureau of Land Management and the USDA Natural Resources Conservation Service. What can you do with this layer? This layer can be used to create maps and to visualize the underlying data. This layer can be used as an analytic input in ArcGIS Desktop. This layer is part of a larger collection of landscape layers that you can use to perform a wide variety of mapping and analysis tasks. The Living Atlas of the World provides an easy way to explore the landscape layers and many other beautiful and authoritative maps on hundreds of topics.

不透水面（impervious surface）指无法让水渗透通过的地表。此类地表的实例包括高速公路、停车场、屋顶以及机场跑道。不透水面不会让雨水渗入土壤，反而会使水在地表聚集并形成径流。不透水面面积的增加会导致需由雨水管理系统处理的水量上升。随径流移动的污染物会先聚集在不透水面上，随后随径流排入溪流与海洋。径流无法进入地下水位，这可能引发其他管理问题，例如基流中断。

美国国家土地覆盖数据库（National Land Cover Database, NLCD）不透水面图层将美国境内每一个30米像元的建成地表中，不透水面所占的百分比作为城市不透水面的表征。

### 数据集摘要
**映射现象**：景观中不透水面的占比。
**时间范围**：美国本土48州的时间序列覆盖2001、2004、2006、2008、2011、2013、2016、2019及2021年；安克雷奇周边的阿拉斯加小部分区域的时间序列为2001、2011及2016年；夏威夷、波多黎各和美属维尔京群岛仅可获取2001年的数据，因此时序中仅包含单幅影像。该数据集可与美国国家土地覆盖数据库土地覆盖图层结合使用。
**单位**：百分比
**像元大小**：30米
**源数据类型**：离散型
**像元类型**：无符号整数
**数据坐标系**：北美阿尔伯斯等积圆锥投影（North America Albers Equal Area Conic, 102008）
**镶嵌投影**：北美阿尔伯斯等积圆锥投影（North America Albers Equal Area Conic, 102008）
**覆盖范围**：美国本土、夏威夷、安克雷奇周边的阿拉斯加部分区域、哥伦比亚特区、波多黎各
**无数据值**：127
**数据源**：多分辨率土地特征联盟（Multi-Resolution Land Characteristics Consortium, MRLC）
**发布日期**：2023年6月30日

### 时间序列
默认情况下，该图层将在客户端中显示时间滑块，支持将时序作为动画播放，动画将逐年推进，但美国本土48州的图层仅在2001、2004、2006、2008、2011、2013、2016、2019及2021年这几个年份发生外观变化。若仅需选择时序中的某一年份，首先需在时间滑块上关闭时间序列功能，随后对图层创建定义查询，仅筛选所需年份的数据。

### 时间序列描述符
MRLC随该不透水面图层发布了一组配套栅格，用于表征每个像元成为不透水面的原因。该配套图层名为“建成不透水面描述符”，当前尚未在该地图服务中上线。描述符图层可识别每个不透水面像元对应的道路类型、核心城区以及能源生产场地，以便对建成地表特征开展更深入的分析。用户可直接从MRLC下载该描述符图层，并添加至ArcGIS Pro中。

### 阿拉斯加、夏威夷与波多黎各
目前阿拉斯加、夏威夷与波多黎各的数据集采用了不同的生成方法，无法像美国本土48州的时序数据那样直接进行对比。若需分析该阿拉斯加区域最近两个数据年份的变化，请务必使用NLCD 2011至2016年建成不透水面变化栅格。针对夏威夷与波多黎各，仅可从MRLC下载2001年的数据。

### 北美阿尔伯斯投影
世界生活地图集（Living Atlas of the World）中的所有NLCD图层在发布前均会投影至北美阿尔伯斯投影坐标系，这使得美国本土、波多黎各、夏威夷及阿拉斯加可共用同一投影进行服务发布与联合分析。Esri开展的测试表明，投影至北美阿尔伯斯坐标系后，NLCD土地覆盖类别的像元总数与输入时完全一致，但像元位置会发生轻微调整。

### 处理模板
该图层附带两种配色方案：冷色调与暖色调。默认配色为冷灰色调，适用于浅灰色与深灰色的网页地图。若需选择2021年之前的默认暖色调配色方案，请在地图客户端中将处理模板更改为“不透水面暖色调渲染器”。

美国国家土地覆盖数据库产品是由多分辨率土地特征联盟（MRLC）合作开展的项目生成的。MRLC联盟由多个联邦机构合作组成，包括美国地质调查局、美国国家海洋和大气管理局、美国环境保护署、美国农业部、美国林务局、国家公园管理局、美国鱼类及野生动物管理局、土地管理局以及美国农业部自然资源保护服务局。

### 图层用途
该图层可用于制作地图并可视化基础数据，也可作为分析输入项用于ArcGIS Desktop中。该图层属于更广泛的景观图层集合的一部分，可用于开展各类制图与分析任务。世界生活地图集提供了便捷的探索方式，可用于浏览景观图层以及数百个主题的众多优质权威地图。