Dynamic 2d rendering in a 3d world - xna

Question

Ok, so I'm working with my first serious experience in XNA. Here's my current task: Download a file. From that file parse a list of "shapes." Each "shape" contains a list of points. These points are the vertexes of lines to be drawn on a map. In fact, it is a county map of the entire United States, so the number of "shapes" is not trivial. We need to be able to zoom in and out of this map, so it needs to be a 2d rendering in 3d space. I want to know what the best strategy for this would be.
I've tried simply using DrawUserIndexedPrimitives, but the draw function takes way too long to process this.
So then I thought I'd try drawing to a series of RenderTarget2D's in LoadContent, and just saving those textures for drawing in the Draw function. But so far all I can seem to get is a series of purple boxes.
EDIT:
Here's my current code:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.IO;
using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Graphics;
using System.Runtime.InteropServices;
namespace First
{
class FirstShape : DrawableGameComponent
{
const int kSizeMultiplier = 10;
public FirstShape(string inFilePath, Game inGame) : base(inGame)
{
DrawOrder = 1000;
mFile = inFilePath;
}
protected override void LoadContent()
{
mSpriteBatch = new SpriteBatch(GraphicsDevice);
FirstMemoryStream stream = FirstMemoryStream.MakeSeekableStream(File.OpenRead(mFile));
// skip headers
stream.Seek(100, 0);
for (int i = 0; stream.Position != stream.Length; i++)
{
stream.Seek(stream.Position + 12, 0);
double minX = stream.ReadDouble();
double minY = stream.ReadDouble();
double maxX = stream.ReadDouble();
double maxY = stream.ReadDouble();
int numParts = stream.ReadInt();
int numPoints = stream.ReadInt();
VertexPositionColor[] points = new VertexPositionColor[numPoints];
stream.Seek(stream.Position + (4 * numParts), 0);
int top, bottom, left, right;
float x2, y2;
x2 = (float)stream.ReadDouble();
y2 = (float)stream.ReadDouble();
Vector2 projectedPoint = Vertex(x2, y2);
left = right = (int)Math.Round(projectedPoint.X * kSizeMultiplier);
top = bottom = (int)Math.Round(maxY - (projectedPoint.Y * kSizeMultiplier));
points[0].Position.X = left;
points[0].Position.Y = top;
for (int j = 1; j < points.Length; j++)
{
float x1 = x2;
float y1 = y2;
x2 = (float)stream.ReadDouble();
y2 = (float)stream.ReadDouble();
Vector2 p1 = Vertex(x1, y1);
Vector2 p2 = Vertex(x2, y2);
p1.X *= kSizeMultiplier;
p1.Y *= kSizeMultiplier;
p2.X *= kSizeMultiplier;
p2.Y *= kSizeMultiplier;
points[j].Position.X = (int)Math.Round(p2.X);
points[j].Position.Y = (int)Math.Round(maxY - p2.Y);
if (points[j].Position.X < left)
left = (int)points[j].Position.X;
if (points[j].Position.X > right)
right = (int)points[j].Position.X;
if (points[j].Position.Y < top)
top = (int)points[j].Position.Y;
if (points[j].Position.Y > bottom)
bottom = (int)points[j].Position.Y;
}
if (mTopLeft.X == 0 || mTopLeft.X > left)
mTopLeft.X = left;
if (mTopLeft.Y == 0 || mTopLeft.Y > top)
mTopLeft.Y = top;
for (int j = 0; j < points.Length; j++)
{
points[j].Color = Color;
}
int width = (right - left) + 1;
int height = (bottom - top) + 1;
mTextures.Add(new FirstImage(GraphicsDevice, width, height, new Vector2(left, top)));
GraphicsDevice.SetRenderTarget(mTextures.Last());
GraphicsDevice.Indices = new IndexBuffer(GraphicsDevice, IndexElementSize.SixteenBits, points.Length, BufferUsage.None);
GraphicsDevice.SetVertexBuffer(new VertexBuffer(GraphicsDevice, VertexPositionColor.VertexDeclaration, points.Length, BufferUsage.None));
BasicEffect basicEffect = new BasicEffect(GraphicsDevice);
basicEffect.LightingEnabled = false;
basicEffect.VertexColorEnabled = true;
foreach (EffectPass pass in basicEffect.CurrentTechnique.Passes)
{
pass.Apply();
GraphicsDevice.DrawIndexedPrimitives(PrimitiveType.LineStrip, 0, 0, points.Length, 0, points.Length - 1);
}
}
GraphicsDevice.SetRenderTarget(null);
stream.Close();
for (int i = 0; i < mTextures.Count; i++)
{
mTextures[i].Position -= mTopLeft;
}
}
public override void Draw(GameTime inTime)
{
mSpriteBatch.Begin();
for(int i = 0; i < mTextures.Count; i++)
{
mSpriteBatch.Draw(mTextures[i], mTextures[i].Position, Color.White);
}
mSpriteBatch.End();
}
private Vector2 Vertex(float inX, float inY)
{
return FirstProjector.Project(new Vector2(inX, inY));
}
public Color Color { get; set; }
private string mFile;
private List<FirstImage> mTextures = new List<FirstImage>();
private SpriteBatch mSpriteBatch;
private Vector2 mTopLeft = new Vector2(0.0f, 0.0f);
private Vector2 mBottomRight = new Vector2(0.0f, 0.0f);
}
class FirstImage : RenderTarget2D
{
public FirstImage(GraphicsDevice inDevice, int inWidth, int inHeight, Vector2 inPosition) : base(inDevice, inWidth, inHeight, false, SurfaceFormat.Color, DepthFormat.None)
{
Position = inPosition;
}
public Vector2 Position {get; set;}
}
}