Topographic Analysis of the Sierra Nevada Mountains Where does the water go?

Transcription

1 UW Hydro Computing Workshops. Jan 27, 2014 Topographic Analysis of the Sierra Nevada Mountains Where does the water go? Modified from Jon Harvey - Earth 176, Fall 2013, UCSB Objectives: Learn to download, mosaic, and analyze topographic data to define watersheds Learn to combine raster datasets to generate additional datasets Deliverables: -1 PDF map, ed as attachments to wamplerd@uw.edu Exercise There are many pages in this document worry not! Only the first part is the assignment! Screenshots are added to the bottom as visual references if you get stuck on a step. Bolded and blue parts of this document show you major steps, and blue parts show you exactly when you should be doing a command in ArcMap that is required for progress. The rest of the text is (mostly) just elaboration and explanation. For this exercise, you will analyze the topography of the Sierra Nevada, one of the more impressive mountain ranges in the US. You ll start by downloading raw elevation data from the web, mosaic it into a useable form, then begin to derive higher-level information about the topography in the study area (such as slope, drainage pathways, watershed boundaries, estimated annual river discharges, ). So, let s dive in! *It is important to remain organized with the files you generate while working in ArcMap. We recommend creating a folder for ArcMap files on an external/flash drive where you have plenty of extra space. Remember that ArcMap runs fastest when the files it uses are on the same computer the software is on so you may end up moving files between locations from time to time. For now, create a folder called Hydro Lab GIS. This is the folder where you ll put the data that you download from the internet and save all the rasters and shapefiles that you generate along the way. *Download the SRTM DEM (Digital Elevation Model) we will use for this lab. To download the SRTM elevation data from the official website, navigate to this URL: However, since the SRTM website is not user friendly, I have done this myself and uploaded them for you to directly download. How nice! The files you will end up with are displayed to the right. Download the files here: 1

2 *You should end up with the following files: srtm_12_04.zip, srtm_12_05.zip, srtm_12_06.zip, srtm_13_04.zip, srtm_13_05.zip, srtm_13_06.zip *Drag the.zip archives into your Hydro Lab GIS folder on your drive if they are not already there. Unzip them all by selecting them all, right-clicking, and using the 7-zip option to Extract here (this may create multiple folders). If it unzips to the same folder, go ahead and overwrite the readme files. We don t care about those in this case. Each folder contains one 5-deg by 5-deg tile of SRTM (Shuttle Radar Topography Mission) elevation data. The SRTM dataset, acquired in 2002, gave the world its firstever (near) global DEM. The raw elevation data from the satellite has been filtered, patched, and processed into its present version (Version 4, released in 2008). It has a nominal resolution of 90 m, which means each pixel, or cell, in the gridded dataset represents a 90m x 90m square of the Earth s surface. Although other attempts at a global DEM have been made (included a recent 30m global DEM), the SRTM dataset remains commonly used and very useful for many applications. *Go ahead and add these files into ArcMap. *Once ArcMap is open, click on Add Data (or find this option in the File toolbar. *Next, click on Connect To Folder. This lets you select a location on a connected drive that ArcMap will know to look for files in the future. *Next, find the folder you created for this lab. Select it, then press OK *Now this folder has been connected! You should see the SRTM files you unzipped (or their respective folders) here. *Now check the spatial reference (a.k.a. coordinate system) of the DEM tiles. Remember, you can find this information in the Layer Properties Source tab. What coordinate system are they in? Are they in a GCS (Geographic Coordinate System) or a PCS (Projected Coordinate System)? If things went smoothly, they should show that they are in WGS84, which is a GCS. It s best to work with DEMs after they ve been projected, as quantitative analyses are much simpler when the horizontal unit is in meters (rather than degrees). However, we don t want to waste time projecting each tile individually. Mosaicing will combine your 6 separate DEMs into a single DEM. Useful! *Mosaic the 6 tiled DEMs into a single DEM, open ArcToolbox Data Management Tools Raster Raster Dataset Mosaic to New Raster. Add all 6 SRTM rasters, set the output directory as your lab folder, and name the output file SRTM_mos. Set the pixel type to 16_bit_signed (learn why we do this in the help panel in the tool window). Set the number of bands to 1 (it will almost always be 1, unless you re dealing with multi-band data like satellite imagery or air photos). Leave the default value for everything else. Hit OK to start the tool. 2

3 After a minute or two, the finished, mosaicked DEM should show up in your map window and the TOC (Table of Contents). Now you have one mega-dem covering the whole area, which is great, because now you don t have to operate on one tile at a time when doing DEM analyses. *Check the mosaic s coordinate system. It should be WGS84, just like the tiles that made it. If you are unsure what a Geographic Coordinate System is, this ESRI link has some helpful information: If you want to know more about Projected Coordinate Systems, the following link has more than you could ever want to know: As a brief overview, GCS tell you where something is on the globe, in degrees. PCS tell you where something is on a map, in meters (or other linear unit). *Go ahead and remove the first six SRTM DEMs from ArcMap. We don t need them anymore. *Now let s try to turn the file s coordinate system from a GCS to a PCS. To project the mosaicked DEM, open ArcToolbox Data Management Tools Projections and Transformations Raster Project Raster. Select the DEM mosaic for the input raster, name the output raster sierra_prj so that you can easily tell it has been projected when you look at the file later. For the output coordinate system, let s think about what projection will work best for our needs. Remember that a common projection for geologists to use is UTM. UTM zones are designed to accurately represent Earth s surface in N-S strips, this projection should contain only minimal distortion for the Sierra, which run N-S. Which UTM zone does it fall in (use the map below to figure it out)? Looks like it falls into both zone 10 and 11, but MAINLY zone 11. So, let s use that one. Select Projected UTM WGS84 Northern Hemisphere WGS 1984 UTM Zone 11N. Set the Resampling Technique to CUBIC, and the Output Cell Size to 90. Your new, projected DEM mosaic should now be in your map view. Compare it to the unprojected one by looking at its extent (Layer Properties Source scroll down). You should see that the unprojected DEM has its extent recorded in degrees, while the projected DEM has its extent recorded in meters away from the UTM origin. *Note: ArcMap handles coordinate systems very well behind the scenes, and does on-the-fly conversions so that your maps stay aligned. Notice how both the mosaic and the projected mosaic show up at the same place? ArcMap displays different coordinate systems into whatever your map (data frame) default coordinate system is (usually defined by the first layer you load into your ArcMap session although you can change them manually anytime you want). Don t get lazy though! It is still a good idea to pay attention to the GCS and PCS your data are in, especially when creating new layers. 3

4 *Give that baby a hillshade! Hillshades are separate layers meant for visualizing your data they make your landscape look more 3-dimensional. Find the hillshade command by going to the ArcToolbox Spatial Analyst Surface Hillshade. Your input raster is sierra_prj and name your output raster sierra_hs. Make sure you are saving your new layers to your lab folder, and not the default geodatabase on the computer (which ArcMap often chooses by default). Remember that if the DEM is projected into a UTM zone, the horizontal units are in meters and the vertical units are in meters. What does that make the z-factor? (Hint: Z- factor is the number of horizontal units in one vertical unit). When you have an unprojected DEM, the horizontal units are in degrees, while the vertical units are in meters, so your z-factor is however many degrees there are in one meter something like for this part of the country you can find conversion tables online. Since our layer is projected, you can leave the Z-factor as 1. You can see one reason (among many) why it is important to know the coordinate system of your data now! *Drape your DEM over its hillshade with some transparency (your preference how much). To do this, re-order the position of your hillshade in the TOC to below your projected DEM layer. Next, go into the layer Properties window for your DEM, go to the Display tab, and add a value into the Transparency text box. * Next, give the DEM an intuitive color ramp. Still in the Layer Properties window, go to the Symbology tab. On the left hand side are different symbol options. For DEMs, stretched works well. Pick a color ramp that you think is good for the Earth s surface. Now go ahead and explore the DEM! If you are feeling adventurous, can you answer the following questions? These aren t required, and you don t need to give us your answers. But they will be useful to think about and help you get used to viewing raster data. *What kind of features can you see in the topography? You should be able to make out mountain ranges, peaks, ridges, fluvial valleys, glacial valleys, fault-controlled valleys, alluvial fans, floodplains, and playas. *Where is the highest elevation in the DEM (hint: look at the color ramp)? How can you find this point on your map (there are several ways raster calculator is one. You may not be able to answer this yet)? Does this location have a name (hint: use the topo basemap, available by clicking on the black triangle on the right side of the Add Data button)? *Where is the lowest point in the DEM? What is its elevation? What is its significance? Do you recognize any other features? 4

5 Using the hydrology tools to define rivers and watersheds One useful geologic application of GIS is for hydrology and surface processes. ArcMap has a suite of tools that are aimed at that application, located within the Hydrology tools within the Spatial Analyst Toolbox. We re going use these tools to define the stream networks that drains the Sierras as well as the watershed boundaries of major rivers in that network. We will use our DEM to predict where rivers 'should' be in the landscape. Knowing that water generally flows downhill, the 'Flow Direction' tool calculates the direction that surface runoff should flow from each pixel in the DEM. The 'Flow Accumulation' tool then uses the flow direction grid to figure out how many pixels lay upstream of every pixel in the grid. At this point we essentially have a stream network; we just need to define a threshold above which a pixel has enough runoff to be considered a 'stream'. *First, let s trim the DEM down to only those areas we need for the analysis (this will save us a lot of time during analyses). Use the Extract by Mask tool (ArcToolbox Spatial Analyst, Extraction Extract by Mask). Your input is sierra_prj, and use the sierramask.shp polygon shapefile that was provided to you as the mask. If you do not have it, this file will be temporarily available here: Save the new DEM as sierra. This should create a new DEM, covering only the area of interest. Check its coordinate system. Is it what it should be (UTM Zone 11)? Go ahead and remove the layers you no longer need from the TOC (at least sierra_mos and sierra_prj) *Before we can run the Flow Direction tool, we have to fix all of the little DEM errors that could complicate later steps. These errors include little depressions or pits in the DEM that would keep water from being able to flow through the digital landscape. Run the Fill tool (ArcToolbox Spatial Analyst Hydrology Fill), setting your DEM ( sierra ) as the input and naming the output 'sierra_f'. Set the z-limit to 50. This variable controls the maximum depth (in meters) of depression that the tool will fill. This means ponds and lakes deeper than 50 layer units (meters in your DEM) will not be filled. This tool may take a while. Sometimes you want to preserve these, and sometimes you want to fill them too. Leaving the z-limit blank will fill every pit. *Calculate Flow Direction, find the 'Flow Direction' tool in the same Hydrology toolbox. Input should be sierra_f, output should be sierra_fd. This will output a grid where each cell s pixel value contains a number representing which directon water flows out of that cell (see diagram to right to interpret which pixel values equate to which flow directions). 5

6 *Calculate Flow Accumulation, use the 'Flow Accumulation' tool, setting the input as your flow direction grid (sierra_fd), and output as sierra_fa. Leave the weight raster blank, and change the Output data type to INTEGER. This tool will probably take a while, so don t cause Arc to freeze by clicking a bunch while it s running. OK, you just created a raster grid whose value at every pixel equals the number of pixels that lay upstream. That number isn t too meaningful in the real world though. A more useful value would be square kilometers. There are 1 million square meters in a square kilometer. To convert the flow accumulation pixel values to represent upstream area in square kilometers rather than pixels, we'll have to do what is called 'Raster Calculator'. This is an incredibly useful tool that allows you to change the values of a raster through mathematical and/or boolean expressions. In this case, we want to convert pixel values into square kilometers. So we first need to multiply the grid by 8100 to convert number of pixels into square meters (remember our cell size is 90 meters). Then we need to divide by 1,000,000 to convert square meters into square kilometers. We can do both of these things in one step by multiplying the grid by *To perform raster math on your flow accumulation grid, use the Raster Calculator by going to Spatial Analyst Tools Map Algebra Raster Calculator. This will bring up a window wherein you can enter the mathematical operation that you'd like to perform on the grid. BEWARE, this tool is very sensitive to how you enter the operation, so don't get frustrated if it freaks out and gives you an error. First, set the output raster as 'sierra_fakm'. Now try this expression: Int(sierra_fa * ) (don't copy and paste...double click the Int expression in the window to the right (this forces the output raster to have integer values). Then double-click sierra_fa to add it to the expression, then use the buttons in the tool window to enter the rest). Save your output as sierra_fakm. See below for example. 6

7 This should have created a new grid that looks a lot like the old one. Now is a good time to remove unnecessary layers to keep your computer from getting bogged down. Remove everything so that you only have sierra_hs and sierra_fakm in the TOC, with the flow accumulation grid on top. In nature, most stream channels start at a drainage area of 1-5 km 2. We don't want to go too small, though, because then we'll have so many streams we won't be able to see anything else in our map. So this is a good time to preview how many streams each threshold option will generate. *Change the display of the flow accumulation grid, go into the symbology for that grid, and select 'classified'. Change the number of classes to 2. What this does is tell ArcMap to display all pixels within one range as one color, and all those outside of that range as another. This will help us because we can set the stream threshold as the classification boundary, such that all pixels below that threshold appear as 'no color', and all pixels above will display as a color of your choice (this can also be useful for highlighting specific values in a raster, such as the highest elevations). *To set the classification boundary, click Classify. This will bring up a histogram of data values (mine range from 0 to 43466). You can change the classification boundaries by dragging the vertical blue lines or by clicking on the numeric break value to the right and typing in the new value. Let s try a break value of 50 km 2 (see example at right). Hit OK, change the symbology for the 0-50 class to None, and for the >50 class to blue or red or something that will stand out over the hillshade. Hit OK. Zoom in on different areas of the map to see where streams are generated using the current threshold of 50 km 2 (they ll be hard to see when you re all zoomed out). You should notice that the tool does a good job creating streams where they should be in valley bottoms. Note that many valley-bottom areas (e.g. the central valley of California) have areas with a number of straight streamline. Once you are satisfied with the number of streams that show up (we ll assume you choose 50 km 2 here, but it s up to you), it's time to export your flow accumulation grid as a raster where ONLY the stream network has a pixel value, and everything else has a value of 'NoData'. This is a necessary step before we can turn the streamlines into a shapefile, which would be beneficial for symbology and other purposes. 7

8 *Set all the non-stream pixels to NoData, open up the raster calculator. Set the output raster to streams_50. This time, you will use the following expression: SetNull("sierra_fakm" < 50, "sierra_fakm") Rather than copy/paste or type in this command, use the buttons in Raster Calculator. Double-click SetNull, and then use the buttons to enter the rest. What this expression does is tell Arc to set all pixels with drainage area <50 to NoData, and where drainage area is >50, leave the value equal to the drainage area. Remember, this does not edit the data you already have - this will only create a new raster layer according to the calculation you set. So you don t have to worry about ruining the data you ve already created if anything goes wrong. This should create a new raster where pixels that fall on a stream have an integer value equal to the upstream drainage area (in km 2 ), and all other pixels have a value of NoData. Test whether it worked by using the identify tool (blue circle with an i in it, located in the toolbars at the top of ArcMap) and querying the pixel value of various on- and off-stream pixels. Remember to turn layers in the TOC on and off so it is easy to see what you are looking at! Now let s work on visualizing the streams. *Convert the stream raster to a shapefile, open up ArcToolbox Spatial Analyst Tools Hydrology Stream to Feature. Input should be the file you just created (sierra_strm50), and the flow direction raster should be sierra_fd - name the output sierra_streamlines. Leave simplify polygons checked. Hit OK. This tool will convert your stream raster file to a vector shapefile. This new shapefile should show up in your TOC. Take a look at the attribute table. The main column of interest is the GRID_CODE column, which contains the drainage area information from the flow accumulation grids. The default is for the streams to show up with a single symbol. Let s symbolize it based on that drainage area column. 8

9 *Symbolize the streams file by drainage area, go to Symbology Quantities Graduated symbols. Set the value field to GRID_CODE. A box should pop up complaining that the maximum sample size has been reached or some such. This means that there are more records in the attribute table than Arc was prepared to scan through. Fix this by clicking Classify Sampling, where you can change the maximum sample size to (this value needs to be slightly bigger than the number of rows in the shapefile s attribute table). Change the breaks between values to 1000, 5000, 10000, and (the last break value should always be the highest value in the dataset). The new appearance of the streams shapefile should help with identifying those higherorder stream channels that collect water from large regions of the Sierra. This layer may take a few seconds to draw, but it should help you identify the trunk drainages that drain the sierra. Now we ll use ArcMap to calculate watersheds for the major rivers. This process involves making a point shapefile to identify the points in the stream network above which we would like to calculate watershed boundaries, then running a tool to generate said watershed polygons. *Take a deep breath of air you are almost done! *Create the point shapefile defining the watersheds of interest - create an empty point shapefile in ArcCatalog (call this sierra_pourpoints ) in your lab folder, then import the coordinate system from the DEM (WGS84 UTM Zone 11N). Look at the screenshots at the bottom of this document if you don t know how to do this. Start an editing session for that shapefile, creating a template (explanation at the top of the next page) if it won t let you create point features right away. You may need to click on the Create Features button in the Editor toolbar first. *We want to identify the watershed boundaries for the biggest streams draining the Sierra, so now you ll have to place a point on the downstream end of each of those streams (shown to the right). To make sure you include all the largest watersheds, consult this list below to make sure you ve included all of them. In each case, place the points near the downstream end of the river. You must place the markers exactly on the streams or the tool will not work. Zoom in very close to each one to make sure you ve placed it accurately, using the flow accumulation layer (sierra_fakm) to make sure you ve hit a high-flow stream pixel. Save and stop editing once you ve drawn in all 6. 9

10 *Note to create feature templates (which sometimes aren t created automatically, and are required to edit features), right click on a layer Edit Features Organize Feature Templates New Template. Select the layer you want to edit, and then click Finish. You should now be able to edit that layer normally. Pour Points: -Sacramento River (Central Valley, drains northeast Sierra) -Tulare Basin (Central Valley, drains southwest Sierra) -Pyramid lake (in western Nevada, northeast of Lake Tahoe) -Carson sink (in western Nevada, northeast of Lake Tahoe) -Owens River (E. side of Sierra, enters Owens Lake from the north) -San Joaquin River (Central Valley, drains east-central Sierra) (hint: you can use the USA Topo basemap to help you find these rivers/basins) *Convert your pour points shapefile into a raster, open ArcToolbox Spatial Analyst Tools Hydrology Snap Pour Point. Set the input data to sierra_pourpoints, the input accumulation to sierra_fa, and the output raster to sierra_ppts. Set the pour point field to FID. Set the snap distance to 5. Hit OK to run the tool. This will create a raster of only 6 pixels, one on each major river near where you placed the pour point. *Outline the major watershed boundaries of the Sierra, open ArcToolbox Spatial Analyst Tools Hydrology Watershed. Select the pour points raster you just created, the FlowDirection grid, and name the output raster sierra_wsheds. This tool will output a raster file where each pixel belongs either inside or outside of one of the watersheds. It should look something like that to the right. If you re missing a watershed, that means you mis-placed one of the pour points, so you ll have to go back and edit the pour points shapefile, fix the point, then run the last two steps over again. Once all six watersheds show up, we need to convert this raster into a polygon shapefile. NOTE: You may notice that there are several subwatersheds that did not get included. If you look carefully at your streamlines, you would notice that these streams got caught in pits that didn t get filled earlier during the Fill step. This is probably due to the pits being deeper than the z-limit we set. We should expect holes like this when we set a z-limit on the fill command, so it is OK that they are there. 10

11 *Convert the watershed raster to a polygon shapefile, open ArcToolbox Conversion tools From Raster Raster to Polygon. The input should be the watershed raster, output should be sierra_wsheds_poly.shp. This new polygon may possible have more than 6 rows in its attribute table, due to multiple polygons being drawn for the same watershed. If you ended up with 6 rows, skip the rest of this paragraph! If you ended up with more than 6 rows, look at the GRIDCODE column in the attribute table. It should contain an integer that corresponds to the integer that the sierra_ppts raster had for each pour point. Select the six biggest polygons (they should highlight in blue within the attribute table), and export them by right-clicking the layer in the TOC Data Export Data. Name this export sierra_wsheds_poly_clean.shp. Now you should have a polygon shapefile with six rows in the attribute table, one for each watershed *Optional steps only! If you are new to ArcMap, you are probably tired of this by now but if you found it easy, feel free to get some practice with these additional steps. * Add a text field to the attribute table, called Wname, labeling each polygon with the name of the watershed (e.g. carson, pyramid, sacramento, sanjoaquin, tulare, owens etc.). You will need to use the Add Field command from the top left of the attribute table, then start an Editing session, return to the attribute table, and enter in the watershed names by hand. Remember to save edits and exit editing when done. *To calculate the drainage area in each watershed, create a new long integer field in the attribute table of your watershed polygon file called areakm. Use the Calculate Geometry command (right-click the column heading) to fill in the cells with area in square kilometers. *Now export the polygons one at-a-time so that you have individual shapefiles for each watershed. (hint: select the watershed, right-click the layer, Data Export Data) Now that we have successfully outlined and calculated the area of the major Sierra watersheds, let s bring in some precipitation data so that we can look at how much precipitation falls on each watershed in an average year. *Add the PRISM precipitation grid to your ArcMap. This file is included in the dropbox link provided earlier in this document. PRISM data is one of the most-used, freely-available precipitation datasets in the US. It is a raster dataset wherein the pixel values represent average annual precipitation, in hundredths of a millimeter (for some strange reason). Notice that the appearance of the dataset resembles that of a DEM that s because the authors of the PRISM dataset incorporate the orographic effect (the tendency for precipitation to increase with elevation during a given storm) into their dataset. Thus, we should expect to see a resemblance to topography in the precipitation data. 11

12 *Use extract by mask to clip the precipitation data to the each of the watershed boundaries. You ll have to run the tool six times, each time producing a new cut-out of the PRISM grid. Name each one wshed_prism, replacing wshed with the abbreviated watershed name you used before. For our purposes, we can use the mean precipitation within each watershed in our calculation of how much water falls into each basin in an average year. *To find the mean value of a raster, go to the layer Properties Source, and scroll down until you see Mean. Write down this number for each precipitation cut-out. Remember, this is the mean annual precipitation in hundredths of a millimeter. Now you should have information about the drainage area and average precipitation of each watershed. This is enough information to calculate the volume of water entering each watershed as precipitation on an annual basis. *To calculate the volume of water that falls into each basin in an average year, you must first convert the mean precipitation amounts into km (since our drainage areas are in km 2 ) by dividing by 100,000,000 (to convert from hundredths of a mm to km). Multiply the mean precipitation by the drainage area, and you should get the total volume of water that falls on each watershed in an average year in km 3. DELIVERABLE You must turn in a single 8.5 x 11 PDF map summarizing your findings: Required: *A map of the watersheds you created *Under the watersheds layer, the original hillshade of the entire region *A scale bar *A north arrow *A legend *A title, including your name! Optional: *Add a colored and partially transparent DEM to drape over your hillshade *Include your streamlines *Re-label your legend so that the text would make sense to any random person -Avoid internal names, like sierra_prj or sierra_wsheds -Instead, use common names, like elevation map or sierra watersheds -Avoid numbers without units, like 1000, 5000, 10000, Instead, define units, either in the legend label or after the numbers: 1000 km^2 -Remove data the user doesn t need to know from the legend, like the hillshade *Try to remove white space (unused map area) from your final map * Add text boxes with each watershed s name *Anything else you think looks nice! 12

13 Send this PDF as an attachment to Remember, key topics covered in this lab are how to see the basic information of your data (such as the projection), how to run tools on raster data, how to use the raster calculator, and how to create new shapefiles. These are things you will use often if you ever become heavily involved with GIS work. Congratulations! You re done! We hope you learned something you will eventually find useful. Remember we have only scraped the surface of what ArcGIS is capable of, but whether it is worth learning or not us up to you and your research questions! 13

14 Below are screenshots of steps in this lab, in roughly chronological order. If you run into trouble, refer to these to see if it explains where to go to get something done. If that still doesn t help, either look for a solution online or ask your amazing workshop leaders what to do! 1) These show how to add data into an ArcMap session 14

15 Once here, just select the layers you want (even multiple at once), and press Add 15

16 2) Below is what you should see after you add the 6 SRTM DEMs to ArcMap. Notice that the colors of the DEMs don t match up at their borders this is expected, since the default color scheme is determined automatically by ArcMAP based on the values within each DEM (which are not identical between locations of course). Nonetheless, the underlying elevation values DO match up at the borders between these DEMs (feel free to check!). 16

17 3) This is what you should see when you are mosaicking your 6 DEMs 17

18 4) This should be the result of your mosaicking. As a nice side bonus, it fixes the issue we had with colors not matching up at DEM boundaries (since those boundaries no longer exist). 18

19 5) This should be what you see when projecting your data 19

20 6) This is what a colored and partially transparent DEM looks like after being draped over a hillshade. Using a hillshade to improve how a DEM map looks is very common, and often a necessity if you want to understand a landscape s shape visually. 20

21 7) Your DEM after using the Extract by Mask tool. 21

22 8) Your Flow Direction raster. Sometimes these come out colored. It s fine if these look bad, this isn t the sort of data that is meant to be visualized. It s very useful for additional processing though! 22

23 9) Your Flow Accumulation raster. Often, this will be very hard to make any sense of when you are zoomed out. If you zoom in on a location, it should become much easier to see the difference between high accumulation (channels) and low accumulation zones. 23

24 10) How to access Symbology to highlight larger values in your Flow Accumulation raster 24

25 11) What you should end up with. It s looking better bit by bit, but we still have work to do. 25

26 12) What you should see after turning your flow accumulation raster into a shapefile (after using the Raster Calculator to set small values to null ). 26

27 13) And what you should see after changing your Symbology for the layer. Much better! 27

28 14) How to create a new shapefile in ArcCatalog. You can do this in ArcCatalog separately from ArcMap, or within the ArcCatalog window within ArcMap. 28

29 15) Defining the coordinate system of the new layer. Sometimes you can import this from another layer, which makes things easy. If not, you have to track down the coordinate system manually, like you did earlier in this lab. 29

30 16) What you should see just before finalizing the creation of your new shapefile 30

31 17) Now we need to return to ArcMap to start an editing session so we can add data to our shapefile 31

32 18) If everything goes well, you can just select sierra_pourpoints from a window that pops up. If it isn t there, then you either need to add it to ArcMap (if you haven t), or create a Feature Template for it. 32

33 19) You may need to click this Create Features button to open the editing window you need. Once you do that, you can just click on the layer you want to edit features for (sierra_pourpoints). It should automatically let you start clicking to add new points, but if not there is a Point button in the Editor toolbar that will let you do this. If you accidently create a point you don t want, you can undo, or select and delete it easily. Here is an example of where you might place one of your pourpoints directly over a channel pixel, somewhere near the end. You can snap pourpoints to channels with a tool if they are offset, but placing them well to begin with saves us that step (and is easy since we only have to place 6 of them!) 33

34 20) You should end up with something that looks like this. It s not very pretty, but we re not worried about pretty right now. Notice the dot associated with each watershed these are roughly where your pourpoints should end up. 34

35 21) Your final map might look something like this. To change the text on the legend, you can simply change the display name of your layers in ArcMap: click on a name in the TOC to edit that text, or remove it entirely to remove that heading from the legend (no worries this doesn t change anything inherent to the data itself!). You could certainly make this look better with more time spent on it, but for now, we re pretty happy with it! 35