Cg Raster Data

[Gabby Dreher]

Thisepisode introduces the two primary types of geospatial data:rasters and vectors. After briefly introducing these data types, thisepisode focuses on raster data, describing some major features and typesof raster data.

The Rfor Raster and Vector Data lesson will focus on how to work withboth raster and vector data sets, therefore it is essential that weunderstand the basic structures of these types of data and the types ofdata that they can be used to represent.

A map of elevation for Harvard Forest derived from the NEONAOP LiDAR sensor is below. Elevation is represented as continuousnumeric variable in this map. The legend shows the continuous range ofvalues in the data from around 300 to 420 meters.

The map above shows the contiguous United States with landcover ascategorical data. Each color is a different landcover category. (Source:Homer, C.G., et al., 2015, Completion of the 2011 National Land CoverDatabase for the conterminous United States-Representing a decade ofland cover change information. Photogrammetric Engineering and RemoteSensing, v. 81, no. 5, p. 345-354)

The map above shows elevation data for the NEON Harvard Forest fieldsite. We will be working with data from this site later in the workshop.In this map, the elevation data (a continuous variable) has been dividedup into categories to yield a categorical raster.

The spatial extent is the geographic area that the raster datacovers. The spatial extent of an R spatial object represents thegeographic edge or location that is the furthest north, south, east andwest. In other words, extent represents the overall geographic coverageof the spatial object.

Raster data can come in many different formats. For this workshop, wewill use the GeoTIFF format which has the extension .tif. A.tif file stores metadata or attributes about the file asembedded tif tags. For instance, your camera might store atag that describes the make and model of the camera or the date thephoto was taken when it saves a .tif. A GeoTIFF is astandard .tif image format with additional spatial(georeferencing) information embedded in the file as tags. These tagsshould include the following raster metadata:

A raster can contain one or more bands. One type of multi-band rasterdataset that is familiar to many of us is a color image. A basic colorimage consists of three bands: red, green, and blue. Each bandrepresents light reflected from the red, green or blue portions of theelectromagnetic spectrum. The pixel brightness for each band, whencomposited creates the colors that we see in an image.

Hi. I have a 2016 NLCD raster layer that I selected certain NLCD Cover Classes from. I wanted to export the selected items to a new raster. After I select the cells, I then go to Data>Export Raster and export. However, the new raster is the same as the previous raster.

In the previous topics we have taken a closer look at vector data. While vectorfeatures use geometry (points, polylines and polygons) to represent the realworld, raster data takes a different approach. Rasters are made up of a matrixof pixels (also called cells), each containing a value that represents theconditions for the area covered by that cell (see figure_raster). In this topicwe are going to take a closer look at raster data, when it is useful and when itmakes more sense to use vector data.

Using raster data is a solution to these problems. Many people use raster data asa backdrop to be used behind vector layers in order to provide more meaningto the vector information. The human eye is very good at interpreting images andso using an image behind vector layers, results in maps with a lot more meaning.Raster data is not only good for images that depict the real world surface (e.g.satellite images and aerial photographs), they are also good for representing moreabstract ideas. For example, rasters can be used to show rainfall trends over anarea, or to depict the fire risk on a landscape. In these kinds of applications,each cell in the raster represents a different value e.g. risk of fire on a scaleof one to ten.

Raster data can be obtained in a number of ways. Two of the most common ways areaerial photography and satellite imagery. In aerial photography, an aeroplaneflies over an area with a camera mounted underneath it. The photographs are thenimported into a computer and georeferenced. Satellite imagery is created whensatellites orbiting the earth point special digital cameras towards the earthand then take an image of the area on earth they are passing over. Once the imagehas been taken it is sent back to earth using radio signals to special receivingstations such as the one shown in figure_csir_station. The process of capturing raster data froman aeroplane or satellite is called remote sensing.

In other cases, raster data can be computed. For example an insurance company maytake police crime incident reports and create a country wide raster map showinghow high the incidence of crime is likely to be in each area. Meteorologists(people who study weather patterns) might generate a province level raster showingaverage temperature, rainfall and wind direction using data collected from weatherstations (see figure_csir_station). In these cases, they will often use rasteranalysis techniques such as interpolation (which we describe in TopicSpatial Analysis (Interpolation)).

Sometimes raster data are created from vector data because the data owners wantto share the data in an easy to use format. For example, a company with road,rail, cadastral and other vector datasets may choose to generate a raster versionof these datasets so that employees can view these datasets in a web browser.This is normally only useful if the attributes, that users need to be aware of,can be represented on the map with labels or symbology. If the user needs to lookat the attribute table for the data, providing it in raster format could be a badchoice because raster layers do not usually have any attribute data associatedwith them.

Every raster layer in a GIS has pixels (cells) of a fixed size that determine itsspatial resolution. This becomes apparent when you look at an image at a smallscale (see figure_raster_small_scale) and then zoom in to a large scale (seefigure_raster_large_scale).

In raster data that is computed by spatial analysis (such as the rainfall map wementioned earlier), the spatial density of information used to create the rasterwill usually determine the spatial resolution. For example if you want to createa high resolution average rainfall map, you would ideally need many weatherstations in close proximity to each other.

If you take a colour photograph with a digital camera or camera on a cellphone,the camera uses electronic sensors to detect red, green and blue light. When thepicture is displayed on a screen or printed out, the red, green and blue (RGB)information is combined to show you an image that your eyes can interpret. Whilethe information is still in digital format though, this RGB information is storedin separate colour bands.

Because having images containing multiple bands of light is so useful in GIS,raster data are often provided as multi-band images. Each band in the image islike a separate layer. The GIS will combine three of the bands and show them asred, green and blue so that the human eye can see them. The number of bands in araster image is referred to as its spectral resolution.

If an image consists of only one band, it is often called a grayscale image.With grayscale images, you can apply false colouring to make the differences invalues in the pixels more obvious. Images with false colouring applied are oftenreferred to as pseudocolour images.

Another approach is to use advanced computer programs to automatically extractvector features from images. Some features such as roads show in an image as asudden change of colour from neighbouring pixels. The computer program looks forsuch colour changes and creates vector features as a result. This kind offunctionality is normally only available in very specialised (and often expensive)GIS software.

Sometimes it is useful to convert vector data into raster data. One side effectof this is that attribute data (that is attributes associated with the originalvector data) will be lost when the conversion takes place. Having vectorsconverted to raster format can be useful though when you want to give GIS datato non GIS users. With the simpler raster formats, the person you give the rasterimage to can simply view it as an image on their computer without needing anyspecial GIS software.

There are a great many analytical tools that can be run on raster data whichcannot be used with vector data. For example, rasters can be used to model waterflow over the land surface. This information can be used to calculate wherewatersheds and stream networks exist, based on the terrain.

Raster data is also very important for disaster management. Analysis of DigitalElevation Models (a kind of raster where each pixel contains the height abovesea level) can then be used to identify areas that are likely to be flooded.This can then be used to target rescue and relief efforts to areas where it isneeded the most.

I get the Export Raster Data dialog box as expected. I would like to export the Raster Data to a GeoTIFF w spatial data brought along. However I cannot change the format box to GeoTIFF. It is greyed out and I am unable to change it.

I am sure something is wrong with my basic approach as I am not an experienced user. But I am following the directions in the help and they are not working. Any advice would be appreciated! Please see attached screenshot.

Ok. I was able to change the format to TIFF and put a file name of DOQQ-Clip.tif in the file name field. However when I use the browse button next to location I am unable to just put a plain folder/directory in it. Please see attached screenshot. NOt sure how to get the TIFF exported still.

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