CHAPTER SEVEN GEOPROCESSING TOOLS

INTRODUCTION Reality > Conception > Representation > Analysis > Documentation > Storage > Distribution Geoprocessing is the ANALYSIS portion No project is complete without the use of several geoprocessing tools Geoprocessing tools can be broken into five categories: overlay and proximity analysis: examines spatial relationships between two datasets answering questions such as "What is near what?" and "How far is something from something else?"; surface analysis: tools that create layers of continuous data such as deriving a slope layer or an aspect layer from a DEM; statistical analysis: examine both spatial and non-spatial statistical relationships utilizing the basic geographic principal that object near each other are more likely to be similar then objects that are far apart; table analysis: finds answers based on table data, and; extraction analysis: tools which create smaller datasets from larger ones.

OVERLAY ANALYSIS The name comes from a time where cartographers used clear sheets to overlay layers and solve spatial problems Overlay Analysis answers questions like: "Does this feature lie inside or outside of another feature?“ "Do these roads start or end on public land?“ "How many feet of river land inside the water district's jurisdiction?". Lets look at a few tools: Erase Intersect Merge Union

ERASE Erase works like an eraser, removing from one layer everywhere another layer lies on top, Uses a binary input model - that is, the tool only accepts two layers: the input (the eraser) and the erased feature (the one to be erased) Erase is used when you want to answer the question: "What lies outside the input area?"

THE “TOP SIX” TOOLS Found in the geoprocessing menu, the “top six” tools are the most commonly used tools in ArcGIS They are: Buffer Clip Intersect Union Merge Dissolve

INTERSECT Intersect looks for geometries which are common to all of the input layers. In contrast to erase, which uses a "binary" input model, intersect uses a multiple input model, meaning the tool will find the areas common to all the layers. Intersect is used when you want to answer the question: "Where are the places the input layers have in common?"

MERGE Merge will create one data set from two, with the output layer containing all the attributes from all of the input layers. No other “geoprocessing is done on the layers” Use when you have two or more that you’d like to smoosh together Use Append when you’d like to add more features to an already existing layer

UNION Union, like Merge combines two or more data sets the difference being Union will compare the relationship between the two, adding fields to the attribute table and splitting geometric features to reflect this relationship.

PROXIMITY ANALYSIS Proximity Analysis tools are another set designed to examine spatial relationships, answering questions such as: "How far is this feature from this other feature?" "How many homes fall within this particular fire protection district?" "What is the most cost efficient delivery area based upon our delivery fee?" "How long does it take to drive from Denver to Eldora Ski Resort?"

PROXIMITY ANALYSIS Proximity Analysis tools are another set designed to examine spatial relationships, answering questions such as: "How far is this feature from this other feature?" "How many homes fall within this particular fire protection district?" "What is the most cost efficient delivery area based upon our delivery fee?" "How long does it take to drive from Denver to Eldora Ski Resort?"

BUFFER Buffer is probably the most common tool you will run This tool creates a “buffer” around the input features as a specified distance It is often combined with other tools such as Select by Location Input feature. In this case a line feature. Measure away from each vertex the designated distance. "Connect the dots" to produce the new output polygon layer

NEAR One common question we ask in GIS is "What is near what?", and often more specifically, "How near or far is one feature from another?". The Near tool will compare two layers and create a new output layer, complete with a new field in the attribute table expressing the exact distance between features (designated by using the FID of the input and near features).

MAKE ROUTE LAYER Make Route Layer is useful for finding things like: Time and distance from a facility Best route Let's say you've found an old folk's home within the buffer that has several calls a week to a particular ambulance bay at various times of the day. The attribute table also shows the response times you notice when the ambulance responds at 9 am and 5 pm, the response time is 14 minutes compared to 2 pm when the response time is only 9 minutes. Knowing that minutes save lives, you decided to determine if there is a faster route the ambulance can take for certain times of the day. In conjunction with some other tools, Make Route Layer will use a variety of inputs such as traffic flows, travel time per segment (section of road that is independent of other sections, such as a change in speed limit or change in same-direction lane count), and time of day to calculate the best-time route. After your analysis, you discover the ambulance can easily shave off 3 minutes of travel time during peak-traffic if it cuts down just a block earlier. Good job! Thanks, GIS.

RASTER PROXIMITY TOOLS Raster proximity tools work on the basis that the cells in a raster are identical in size

EUCLIDEAN DISTANCE TOOLS Since we know that raster layers can show features such as building, roads, rivers, etc, we can measure from a given feature all the other cells in the image. The Euclidean Distance tool set is used for just this purpose. After defining the feature from which to measure, the tool uses the known cell measurement and returns a new raster layer with a measurement value associated with each cell. Use the Euclidean Distance tool to find the distance from a feature (or features) to other places in the image. Use the Euclidean Allocation tool to assign all the cells in a raster to features based on closest proximity. In other words, the distance of all the cells in a raster is measured from all the designated features in the same raster. Cells are then assigned to a feature based on the shortest distances, or closest proximity. If a distance is defined in the Euclidean Allocation tool (vs. allowing the tool to just do its thing), it is analogous to the vector buffer tool. Use the Euclidean Direction tool to find the direction of each cell from a feature (or features) in a raster

EXTRACTION ANALYSIS Extraction analysis is used to make a larger data set a smaller data set In Chapter Five, we looked at the idea of selecting and exporting value We selected by attribute and by location

CLIP Clip is the “cookie cutter” tool Your input feature is the “cookie cutter” Your clip feature is the “cookie dough” Use Clip to cut out a series of features from one layer based upon the perimeter of another

DISSOLVE Dissolve is an extraction tool in the sense it makes a larger data set smaller (more features to less features) unlike export or clip, the geometric size of the data set remains the same. In other words, the output geometry from tools like clip or export will look differently than the output of dissolve.

SURFACE CREATION Hillshades are used for backgrounds Slope is used determine the percent rise of the hillsides Aspect shows which way the hillside faces (north, east, west, south) HillshadeSlopeAspect

CONTOUR LINES Contour lines express the elevation of the slope

ENABLING EXTENSIONS Each extension in ArcGIS costs money beyond the base price of the software, and many school and companies save money by not purchasing extensions they do not need or only purchasing a limited amount of licenses to be shared on a network of several users. To allow everyone access to all of the licenses, extensions must be enabled before tools within that particular toolbox can be launched. Enabling the extension checks it out from a central location and allows the tools associated with it to be utilized. Think about it like a public library. Instead of purchasing tons of copies of one particular best- seller, they purchase a limited number of copies, then lend them out free of charge to their registered users. While the book is checked out, no one else can read it. ArcGIS extensions work the same way - the extension will only work for the same number of computers for which the company or school has paid for until they are checked back in.