building a network from scratch for network analyst · 2016-11-06 · these network attributes are...
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Building a Network Data set from scratch Building a Network Data set from scratch is challenging. This challenge is usually due to varied quality of the street data that comes as a mapped layer from government agencies. It often takes several iterations (often including building topology to make sure all the lines [streets] are connected) before one can achieve a good network for modeling. This is likely the reason why many in industry purchase prebuilt networks from companies like Navteq. However, it is possible to do this yourself but one never knows how clean the data is that you download from a government server. Here is a link to several streets files and a few prebuilt networks in California: http://ratt.ced.berkeley.edu/downloads/BayAreaStreets/Bay_Area_Streets.html I have included the steps to download and build a network dataset for the county of San_Joaquin, CA. You can input your data where I use Stockton in the text below and build your own. My notes are extremely cryptic here as they are meant to complement the lab 10 assignment. I saved the Transportation network in a geodatabase called SN1Geodatabase.gdb. I include in the picture the two original shape files, one was a county polygon and the other was the transportation network of San_Joaquin_Streets. After I build the network, I load it to ArcGIS, displayed here in the following set of graphics.
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Building your own Network for Network Analyst
Start ArcCatalog
Click Customize ! Extensions
Check Network Analyst
Next …. Create a File Geodatabase
I call this one: SN1Geodatabase.gdb
I then Create a New Feature Dataset
I Call it Transportation
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I set its Projections and Datum. I make it the same as the original San_Joaquin_Streets.shp shapefile.
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I import the shapefile (San_Joaquin_Streets.shp) into the Geodatabase as a Feature Class
and call it Streets .
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I then drag it into the Transportation Feature Dataset
I then add fields to the Streets Feature Class
The Fields and Process of Adding fields to data base:
Call it METERS and make it Floating Point.
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Calculate METERS of each line segment
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Add a field MeterPm to calculate SPEED in Meters/Minute . We want to model in increments of minutes of time traveled. CONVERSION from miles/hour to Meter/minute is needed for this as we are in a projection that deals in METERS and in a city we likely want to model in Minutes rather than Hours.
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Calculate
MeterPm = SPEED * 26.82
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Result looks like this
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Add 2 fields to calculate how long it takes to travel the length of each line segment. These fields are the time to go From node - To node direction, (FT_Minutes)
and the time to go To node - From node direction (TF_Minutes)
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Result looks like this
Calculate FT_Minutes = [METERS] / [MeterPm]
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and then TF_Minutes = FT_Minutes
We do this incase later the time will increase moving in one of the directions.
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Result looks like this
With the fields created we can now create a new Network dataset
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These Network attributes are properties of the network that control navigation. Common examples are cost attributes that function as impedances over the network and restriction attributes that prohibit traversal in both directions or one direction, like one-way streets. Network Analyst analyzes the source feature class (or classes) and looks for common fields like Meters, Minutes (FT_Minutes and TF_Minutes, one for each direction), and Oneway. If it finds these fields, it automatically creates the corresponding network attributes and assigns the respective fields to them. (This can be viewed by clicking the
Evaluators tab.) Click the Meters row to select it, then click Evaluators to examine how the values of network attributes are determined.
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The Evaluators dialog box opens.
The table on the Source Values tab lists the source feature classes. Linear source feature classes, that become edge elements in the network dataset, are listed twice; once for the from–to direction and once for the to–from direction. (The directions are in relation to the digitized direction of the source line feature – just a point of information and not critical to remember but an important part of topology building.)
The Type column shows the type of evaluator used to calculate the network attribute values.
The Value column holds information the evaluator needs to calculate attribute values.
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From the Attribute drop-‐down list, click each type of attribute, one at a time, and inspect the evaluator types and values for the source feature classes.
Here I click Minutes:
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After clicking OK then select Next > to move on
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select yes here….and finally Next >
Finally select Finish to build the Network dataset….
A progress bar opens showing you that Network Analyst is creating the network dataset.
Select Yes
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The new network dataset, Streets_ND, is added to ArcCatalog along with the system junctions feature class, Streets_ND_Junctions.
Preview the network dataset by clicking its name and
clicking the Preview tab.
Close ArcCatalog.
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Open ArcMap and add the New Feature Dataset … Transportation
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Make sure you turn on the Network Analyst extension
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You are ready to Network model…..