Google earth engine（GEE）——绘制直方图histogram

很多朋友可能喜欢在GEE中直接进行某些数据的统计和出图，所以今天我们就来讲讲直方图的统计和绘制。

首先看看主要用到的GEE当中的方法：

ui.Chart.image.histogram(image, region, scale, maxBuckets, minBucketWidth, maxRaw, maxPixels)

Generates a Chart from an image. Computes and plots histograms of the values of the bands in the specified region of the image.

- X-axis: Histogram buckets (of band value).//恒后一般是你的波段

- Y-axis: Frequency (number of pixels with a band value in the bucket).//相应波段的频率

Returns a chart.

Arguments:

image (Image):

The image to generate a histogram from.

region (Feature|FeatureCollection|Geometry, optional):

The region to reduce. If omitted, uses the entire image.

scale (Number, optional):

The pixel scale used when applying the histogram reducer, in meters.

maxBuckets (Number, optional):

The maximum number of buckets to use when building a histogram; will be rounded up to a power of 2.

构建直方图时使用的最大桶数，也就是列数；将四舍五入到 2 的幂。

minBucketWidth (Number, optional):

The minimum histogram bucket width, or null to allow any power of 2.

最小直方条宽度，或 null 以允许任何 2 的幂。

maxRaw (Number, optional):

The number of values to accumulate before building the initial histogram.

在构建初始直方图之前要累积的值的数量。

maxPixels (Number, optional):

If specified, overrides the maximum number of pixels allowed in the histogram reduction. Defaults to 1e6.

如果指定，则覆盖直方图缩减中允许的最大像素数。默认为 1e6

Returns: ui.Chart

此外还有一个功能要说：

setOptions(options)用于设置你要选择的什么图形，并由很多参数可以设置

DEPRECATED: Use ui.Chart.* instead.目前用这个替代

Sets options used to style this chart.

Returns a new Chart with the passed-in options.

Arguments:

this:chart (Chart):

The Chart instance.

options (Object):

An object defining chart style options such as:

- title (string) The title of the chart.

标题（字符串）图表的标题

- colors (Array) An array of colors used to draw the chart. Its format should follow the Google Visualization API's

颜色（数组）用于绘制图表的颜色数组。其格式应遵循 Google Visualization API 的选项：

options: https://developers.google.com/chart/interactive/docs/customizing_charts

可以将下面的颜色换成

colors: ['#e0440e', '#e6693e', '#ec8f6e', '#f3b49f', '#f6c7b6']

还可以设置长宽：

 width: 400,
 height: 240,

Returns: Chart

这次用到的数据主要是DEM数据，而且以矩形的形式圈定了一块范围，最后分别统计不同高程处的影像数量。代码很简单：

var elevation = ee.Image('CGIAR/SRTM90_V4');
var colorado = ee.Geometry.Rectangle({
  coords: [-109.05, 37, -102.05, 41],
  geodesic: false
});

// 生成直方图数据.  设置直方图每一条的宽度是多少，或者是2的N次方

var histogram = ui.Chart.image.histogram({
  image: elevation,
  region: colorado,
  scale: 200,
  minBucketWidth: 300
});
histogram.setOptions({
  width: 400,
  height: 240,
  title: 'Histogram of Elevation in Colorado (meters)'
  colors: ['#e0440e', '#e6693e', '#ec8f6e', '#f3b49f', '#f6c7b6']
});

print(histogram);

Map.addLayer(elevation.clip(colorado));
Map.setCenter(-107, 39, 6);

这是选择的美国科罗拉多州的DEM看起来不清晰，因为没有使用相应的色调进行描绘

换了色彩之后是这样的